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Neigheve Ut he aL ' * 4 i Haee PS tt Re iwh 3 Ci int Fa Wan i i, “eutee ehely ‘ns the aoiEt eae, TD al Tate A eta aasteglat meena aa "el bevad ss MAT Sy «Fi HK beget riba a eae Hie aah bay ik jase: 4h hip cs WMO iat mn cooe rs id a) fay WE 6! 4 i nega neces f Cit st + ’ pet sii He me Pec fee he Svat tye pee), af sie Nie mee Sepak is Ay ‘ Rion he raat ‘ee bay 4 areal r ohajhe erate betes il ot eli Li ais ; ae ee * Wide i 4 f sas Sle. Cane Ran bans Soa prea are, sed peegtiohe e an i tran ie CORNELL UNIVERSITY LIBRARY BOUGHT WITH THE INCOME OF THE SAGE ENDOWMENT FUND GIVEN IN 1891 BY HENRY WILLIAMS SAGE 1924 024 557 641 Cornell University Library The original of this book is in the Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924024557641 GUIDE TO THE STUDY OF FiSHro A GUIDE TO THE STUDY OF FISHES BY DAVID STARR JORDAN President of Leland Stanford Junior University With Colored Frontispieces and 427 Illustrations IN TWO VOLUMES VOL I. ‘¢T am the wiser in respect to all knowl- edge and the better qualified for all fortunes for knowing that there is a minnow in that brook.” — Thoreau NEW YORK HENRY HOLT AND COMPANY 1905 Copyright, 1905 BY HENRY HOLT AND COMPANY Published March, 1g05 ROBERT DRUMMOND, PRINTER, NEW YORK To Theodore Gill, Ichthyologist, Philosopher, Critic, Master in Taxonomy, this volume is dedicated. PREFACE THis work treats of the fish from all the varied points of view of the different branches of the study of Ichthyology. In general all traits of the fish are discussed, those which the fish shares with other animals most briefly, those which relate to the evolution of the group and the divergence of! its various classes and orders most fully. The extinct forms are restored to their place in the series and discussed along with those still extant. In general, the writer has drawn on his own experience as an ichthyologist, and with this on all the literature of the science. Special obligations are recognized in the text. To Dr. Charles H. Gilbert, he is indebted for a critical reading of most of his proof-sheets; to Dr. Bashford Dean, for criticism of the proof- sheets of the chapters on the lower fishes; to Dr. William Emer- son Ritter, for assistance in the chapters on Protochorduta; to Dr. George Clinton Price, for revision of the chapters on lancelets and lampreys, and to Mr. George Clark, Secretary of Stanford University, for assistance of various kinds, notably in the prep- aration of the index. To Dr. Theodore Gill, he has been for many years constantly indebted for illuminating suggestions, and to Dr. Barton Warren Evermann, for a variety of favors. To Dr. Richard Rathbun, the writer owes the privilege of using illustrations from the ‘Fishes of North and Middle America” by Jordan and Evermann. The remaining plates were drawn for this work by Mary H. Wellman, Kako Morita, and Sekko Shimada. Many of the plates are original. Those copied from other authors are so indicated in the text. No bibliography has been included in this work. A list of writers so complete as to have value to the student would make vii Viil Preface a volume of itself. The principal works and their authors are discussed in the chapter on the History of Ichthyology, and with this for the present the reader must be contented. The writer has hoped to make a book valuable to technical students, interesting to anglers and nature lovers, and instruc- tive to all who open its pages. DaviD STARR JORDAN. Pato ALTO, SANTA CLARA COUNTY, CAL., October, 1904. CONTENTS VOL. I. CHAPTER I. THE LIFE OF THE FISH (Lepomis megalotis). What is a Fish?—The Long-eared Sunfish—Form of the Fish.—Face of the Fish.—How the Fish Breathes.—Teeth of the Fish.—How the Fish Sees.— Color of the Fish.—The Lateral Line.—The Fins of the Fish:—The Skele- ton of the Fish.—The Fish in Action.—The Air-bladder.—The Brain of the Byshi=— The Rish'ssNeSts 23, 26 ona. 2nd pSibecer Cala bara ones CHAPTER II. THE EXTERIOR OF THE FISH. Form of Body.—Measurement of the Fish.—The Scales or Exoskeleton.— Ctenoid and Cycloid Scales.—Placoid Scales.—Bony and Prickly Scales. —Lateral Line.—Function of the Lateral Line.—The Fins of Fishes.— IVEISCI ESE nick tee CON sees tamu aS A Dey PUR ORE AOR Renae ee CHAPTER III. THE DISSECTION OF THE FISH. The Blue-green Sunfish.—The Viscera.—Organs of Nutrition.—The Alimen- tary Canal.—The Spiral Valve.—Length of the Intestine... .............. CHAPTER IV. THE SKELETON OF THE FISH. Specialization of the Skeleton.—Homologies of Bones of Fishes.—Parts of the Skeleton. —Names of Bones of Fishes.—Bones of the Cranium.—Bones of the Jaws.—The Suspensorium of the Mandible-—Membrane Bones of Head. —Branchial Bones.—The Gill-arches.—The Pharyngeals.—The Vertebral Column.—The Interneurals and Interhemals.—The Pectoral Limb.—The Shoulder-girdle.—The Posterior Limb.—Degeneration.—The Skeleton in Primitive Fishes. —The Skeleton of Sharks.—The Archipterygium.. . . ix PAGE 16 Xx Contents CHAPTER V. MORPHOLOGY OF THE FINS OF FISHES. PAGE Origin of the Fins of Fishes.—Origin of the Paired Fins.—Development of the Paired Fins in the Embryo.—Evidences of Palaontology.—Current The- ories as to Origin of Paired Fin—Balfour’s Theory of the Lateral Fold.— Objections. —Objections to Gegenbaur’s Theory.—Kerr’s Theory of Modi- fied External Gills.—Uncertain Conclusions.—Forms of the Tail in Fishes. —Homologies of the Pectoral Limb.—The Girdle in Fishes other than Dipnoanss cci.sis veognnckes venice pce Sh mea tees Ganka BSE ERE bh oes 62 CHAPTER VI. THE ORGANS OF RESPIRATION. How Fishes Breathe —The Gill Structures—The Air-bladder—Origin of the Air-bladder.—The Origin of Lungs.—The Heart of the Fish——The Flow EB OO Se sea his ce a ie ee la ch tne aes rane deal shal talc Mbtwans Gh vk Maat eR gl CHAPTER VII. THE NERVOUS SYSTEM. The Nervous System.—The Brain of the Fish—The Pineal Organ.—The Brain of Primitive Fishes —The Spinal Cord.—The Nerves...........--- 109 CHAPTER VIII. THE ORGANS OF SENSE. The Organs of Smell—The Organs of Sight-—The Organs of Hearing.— Voices of Fishes.—The Sense of Taste.—The Sense of Touch............ 115 CHAPTER IX. THE ORGANS OF REPRODUCTION. The Germ-cells.—The Eggs of Fishes.—Protection of the Eggs.—Sexual Modi- HRGALEVO In ee ses eset eset GAG ate secerea tune Sie as ec mee pension acre tas Raat peek aa tee ee 124 CHAPTER X. THE EMBRYOLOGY AND GROWTH OF FISHES. Postembryonic Development.—General Laws of Development.—The Signifi- cance of Facts of Development.—The Development of the Bony Fishes. — The Larval Development of Fishes.—Peculiar Larval Forms.—The Devei- opment of Flounders.—Hybridism.—The Age of Fishes.—Tenacity of Contents Life—Effect of Temperature on Fishes.—Transportation of Fishes.—Re- production of Lost Parts.—Monstrosities among Fishes................... CHAPTER XI. INSTINCTS, HABITS, AND ADAPTATIONS. The Habits of Fishes.—Irritability of Animals.—Nerve-cells and Fibers.— The Brain or Sensorium.—Reflex Action.—Instinct.—Classification of Instincts.—Variability of Instincts—Adaptations to Environment.—Flight of Fishes.—Quiescent Fishes.—Migratory Fishes—Anadromous Fishes.— Pugnacity of Fishes.—Fear and Anger in Fishes.—Calling the Fishes.— Sounds of Fishes.—Lurking Fishes.—The Unsymmetrical Eyes of the Flounder:—Carrying Eggs in the Mouths . y+ 4s scaney eee ey des oe dea ces ne CHAPTER XII. ADAPTATIONS OF FISHES. Spines of the Catfishes.—Venomous Spines.—The Lancet of the Surgeon-fish. —Spines of the Sting-ray.—Protection through Poisonous Flesh of Fishes.— Electric Fishes.—Photophores or Luminous Organs.—Photophores in the Iniomous Fishes.—Photophores of Porichthys.—Globefishes.—Remoras.— Sucking-disks of Clingfishes——Lampreys and Hogfishes.—The Sword- fishes.—The Paddle-fishes—The Sawfishes.—Peculiarities of Jaws and Teeth.—The Angler-fishes.—Relation of Number of Vertebre to Temper- ature, and the Struggle for Existence.—Number of Vertebra: Soft-rayed Fishes; Spiny-rayed Fishes; Fresh-water Fishes; Pelagic Fishes.—Varia- tions in Fin-rays.—Relation of Numbers to Conditions of Life.—Degenera- tion of Structures.—Conditions of Evolution among Fishes............... CHAPTER XIII. COLORS OF FISHES. Pigmentation.—Protective Coloration.—Protective Markings.—Sexual Colora- tion.—Nuptial Coloration.—Coral-reef Fishes.—Recognition Marks.—In- tensity of Coloration.—Fading of Pigments in Spirits.—Variation in Pat- POU M ae sipetenscwan eal ay B Maoh Yo.3 juts SG Ree EE oie Be WIE reeled ele that's CHAPTER XIV. GEOGRAPHICAL DISTRIBUTION OF FISHES. Zoogeography.—General Laws of Distribution.—Species Absent through Bar- riers.—Species Absent through Failure to Maintain Foothold.—Species Changed through Natural Selection.—Extinction of Species.—Barriers xl PAGE 152 179 226 Xll Contents PAGE Checking Movements of Marine Species.—Temperature the Central Fact in Distribution.—Agency of Ocean Currents.—Centers of Distribution. — Distribution of Marine Fishes.—Pelagic Fishes.—Bassalian Fishes.—Lit- toral Fishes.—Distribution of Littoral Fishes by Coast Lines.—Minor Faunal Areas.—Equatorial Fishes most Specialized.—Realms of Distribu- tion of Fresh-water Fishes.—Northern Zone.—Equatorial Zone.—Southern Zone.—Origin of the New Zealand Fauna. ..................555 ee eu CHAPTER XV. ISTHMUS BARRIERS SEPARATING FISH FAUNAS. The Isthmus of Suez.—The Fish Fauna of Japan.—Fresh-water Faunas of Japan.—Faunal Areas of Marine Fishes of Japan.—Resemblance of Japan- ese and Mediterranean Fish Faunas.—Significance of Resemblances.— Differences between Japanese and Mediterranean Fish Faunas.—Source of Faunal Resemblances.—Effects of Direction of Shore Lines.—Numbers of Genera in Different Faunas.—Significance of Rare Forms.—Distribution of Shore-fishes.—Extension of Indian Fauna.—The Isthmus of Suez as a Bar- rier to Distribution.—Geological Evidences of Submergence of Isthmus of Suez.—The Cape of Good Hope as a Barrier to Fishes.—Relations of Japan to the Mediterranean Explained by Present Conditions.—The Isthmus of Panama as a Barrier to Distribution.—Unlikeness of Species on the Shores of the Isthmus of Panama.—Views of Dr. Giinther on the Isthmus of Panama.—Catalogue of Fishes of Panama.—Conclusions of Evermann & Jenkins.—Conclusions of Dr. Hill—Final Hypothesis as to Panama....... 255 CHAPTER XVI. DISPERSION OF FRESH-WATER FISHES. The Dispersion of Fishes.—The Problem of Oatka Creek.—Generalizations as to Dispersion.—Questions Raised by Agassiz.—Conclusions of Cope.— Questions Raised by Cope.—Views of Giinther.—Fresh-water Fishes of North America.—Characters of Species—Meaning of Species.—Special Creation Impossible.-—Origin of American Species of Fishes. ............ 282 CHAPTER XVII. DISPERSION OF FRESH-WATER FISHES. (Continued.) 3arriers to Dispersion of Fresh-water Fishes: Local Barriers.—Favorable Waters Have Most Species.—Water-sheds.—How Fishes Cross Water-sheds. —The Suletind.—The Cassiquiare.—Two-Ocean Pass.—Mountain Chains. —Upland Fishes.—Lowland Fishes.—Cuban Fishes.—Swampy Water- sheds.—The Great Basin of Utah.—Arctic Species in Lakes.—Causes of Dispersion Gull Mi O PeratiOn case Oe ccasmigeod ae aoe osewal yes See ae res Contents Xili CHAPTER XVIII. FISHES AS FOOD FOR MAN. PAGE The Flesh of Fishes.—Relative Rank of Food-fishes—Abundance of Food- fishes.—Variety of Tropical Fishes.—Economic Fisheries.—Angling....... 320 CHAPTER XIX. DISEASES OF FISHES. Contagious Diseases: Crustacean Parasites.—Myxosporidia or Parasitic Proto- zoa.—Parasitic Worms: Trematodes, Cestodes.—The Worm of the Yellow- stone—The Heart Lake Tapeworm.—Thorn-head Worms.—Nematodes. —Parasitic Fungi.—Earthquakes.—Mortality of Filefish................. 340 CHAPTER XX. THE MYTHOLOGY OF FISHES. The Mermaid.—The Monkfish.—The Bishop-fish.—The Sea-serpent ........ 359 CHAPTER XXI. THE CLASSIFICATION OF FISHES. Taxonomy.—Defects in Taxonomy.—Analogy and Homology.—Coues on Classification.—Species as Twigs of a Genealogical Tree.—Nomenclature.— The Conception of Genus and Species.—The Trunkfishes.—Trinomial Nomenclature.—Meaning of Species.—Generalization and Specialization.— High and Low Forms.—The Problem of the Highest Fishes.............. 367 CHAPTER XXII. THE HISTORY OF ICHTHYOLOGY. Aristotle. —Rondelet.— Marcgraf.— Osbeck.— Artedi.— Linnzeus.— Forskal.— Risso.—Bloch.—Lacépéde.—Cuvier.—Valenciennes.—A gassiz.— Bonaparte. —Giinther.— Boulenger.—Le Sueur.— Miiller.—Gi'l.— Cope.— Liitken.— Steindachner.— Vaillant.—Bleeker.—Schlegel.—Poey.—Day.—Baird.—Gar- man.— Gilbert.— Evermann.— Eigenmann.— Zittel— Traquair.— Wood- ward.—Dean.—Eastman.—Hay.—Gegenbaur.—Balfour.—Parker.—Dollo. . 387 CHAPTER XNIII. THE COLLECTION OF FISHES. How to Secure Fishes.—How to Preserve Fishes.—Value of Formalin.—Rec- ords of Fishes.—Eternal Vigilance. ........-. +6200 eee eee reese 429 X1V Contents CHAPTER XXIV. THE EVOLUTION OF FISHES. The Geological Distribution of Fishes——The Earliest Sharks.—Devonian Fishes.—Carboniferous Fishes.—Mesozoic Fishes.—Tertiary Fishes.—Fac- tors of Extinction —Fossilization of a Fish—The Earliest Fishes.—The Cyclostomes.—The Ostracophores—The Arthrodires.—The Sharks.— Origin of the Shark.—The Chimeras.—The Dipnoans.—The Crossopte- rygians.—The Actinopteri—The Bony Fishes. .........----00++ +205 000> CHAPTER XXV. THE PROTOCHORDATA. The Chordate Animals.—The Protochordates—Other Terms Used in Classifi- cation—The Enteropneusta.—Classification of Enteropneusta.—Family Harrimaniide.—Balanoglosside.—Low Organization of Harrimaniide.... . CHAPTER XXVI. THE TUNICATES, OR ASCIDIANS. Structure of Tunicates.—Development of Tunicates.—Reproduction of Tuni- cates.—Habits of Tunicates.—Larvacea.—Ascidiacea.—Thaliacea.— Origin of ‘Tunicatés:—Degeneration of Tunicates. «0 ...0..10 se senreans cade ee CHAPTER XXVII. THE LEPTOCARDII, OR LANCELETS. The Lancelet.—Habits of Lancelets.—Species of Lancelets.—Origin of Lance- CHAPTER XXVIII. THE CYCLOSTOMES, OR LAMPREYS. The Lampreys.—Structure of the Lamprey.—Supposed Extinct Cyclostomes.— Conodontes.—Orders of Cyclostomes——The Hyperotreta, or Hagfishes.— The Hyperoartia, or Lampreys.—Food of Lampreys.—Metamorphosis of Lampreys.—Mischief Done by Lampreys.—Migration or ‘‘Running” of Lampreys.—Requisite Conditions for Spawning with Lampreys.—The Spawning Process with Lampreys.—What Becomes of Lampreys after SPaAWHMe? \. swiss cep sae eee PAGE 435 460 . 467 Contents CHAPTER XXIX. THE CLASS ELASMOBRANCHII, OR SHARK-LIKE FISHES. The Sharks.—Characters of Elasmobranchs.—Classification of Elasmobranchs. —Subclasses of Elasmobranchs.—The Selachii.—Hasse’s Classification of Elasmobranchs. — Other Classifications of Elasmobranchs. — Primitive Sharks.—Order Pleuropterygii—Order Acanthodii.—Dean on Acanthodii. —Order Ichthyotomi................ CHAPTER XXX. THE TRUE SHARKS. Order Notidani—Family Hexanchide.—Family Chlamydoselachide.—Order Asterospondyli.—Suborder Cestraciontes.—Family Heterodontide.—Edes- tus and its Allies.—Onchus.—Family Cochliodontide.—Suborder Galei.— Family Scyliorhinide.—The Lamnoid, or Mackerel-sharks.—Family Mit- sukurinide, the Goblin-sharks.—Family Alopiide, or Thresher-sharks.— Family Pseudotriakide.—Family Lamnida.—Man-eating Sharks.—Family Cetorhinide, or Basking Sharks.—Family Rhineodontide.—The Carcharioid Sharks, or Requins.—Family Sphyrnide, or Hammer-head Sharks.—The Order of Tectospondyli.—Suborder Cyclospondylii-Family Squalide.— Family Dalatiide.—Family Echinorhinide.—Suborder Rhine.—Family Pristiophoride, or Saw-sharks.—Suborder Batoidei, or Rays.—Pristidide, or Sawfishes.—Rhinobatide, or Guitar-fishes.—Rajide, or Skates.—Narco- batide, or Torpedoes. — Petalodontide. — Dasyatide, or Sting-rays. — Myliobatide.—Family Psammodontide.—Family Mobulide.............. CHAPTER XXXI. THE HOLOCEPHALI, OR CHIMAIRAS. The Chimeras.—Relationship of Chimeras.—Family Chimeride.—Rhino- chimeride.—Extinct Chimeroids.—Ichthyodorulites...............-.---. CHAPTER XXXII. THE CLASS OSTRACOPHORI. Ostracophores.—Nature of Ostracophores.—Orders of Ostracophores.—Order Heterostraci.—Order Osteostraci—Order Antiarcha.—Order Anaspida.... 5 CHAPTER XXNIII. ARTHRODIRES. The Arthrodires.—Occurrence of Arthrodires.—Arthrognathi.—Anarthrodira.— Stegothalami.—Arthrodira.—Temnothoraci.—Arthrothoraci.—Relations of XV PAGE 506 XVI Contents Arthrodires. — Suborder Cyclia. — Paleospondylus. — Gill on Paleospon- dylus.—Views as to the Relationships of Palwospondylus: Huxley, Tra- quair, 1890. Traquair, 1893. Traquair, 1897. Smith Woodward, 1892. Dawson, 1893. Gill, 1896. Dean, 1896. Dean, 1898. Parker & Has- well, 1897. Gegenbaur, 1898.—Relationships of Palaospondylus......... CHAPTER XXXIV. THE CROSSOPTERYGII. Class Teleostomi.—Subclass Crossopterygii.—Order of Amphibians.—The Fins of Crossopterygians.—Orders of Crossopterygians.—Haplistia.—Rhipidistia. —Megalichthyide.—Order Actinistia.—Order Cladistia—The Polypte- ASN GLE ieee ta farssehec eae veer a Soe RAE URES ute nan A ea Dey cA rey Ie Ngai Aal ) t ACRca i CHAPTER XXXV. SUBCLASS DIPNEUSTI, OR LUNGFISHES. The Lungfishes.—Classification of Dipnoans.—Order Ctenodipterini.—Order Sirenoidei.—Family Ceratodontida.—Development of Neoceratodus.—Lepi- dosirenidez.—Kerr on the Habits of Lepidosiren. ...................00.. PAGE 581 598 609 LIST OF ILLUSTRATIONS VOL. I. PAGE Lepomis megalotis, Long-eared Sunfish............. 000.00 cece ec eeeueeees 2 Leponus megalotis, Long-eared Sunfish............ 0.00: cece sce cence eee eees 4 Lupomots gibbosus, Common Sunfish. oa accvee ab o0a os sead a Messen age 7 zorthe dictyogramma, a Japanese Blenny...............00.00 cece cece ee eee 9 Eupomotis gibbosus, Common Sunfish... . .... 2.6.6 eee eee eee eee eens 13 Af onocentris japonicus, Pine-cone Fish... 000.00. c0e scenes cen sdaeeesdesaves 16 Loodon Wysiris,. ROrcupinernshy:. 4.2 ssi savas. ctamadaruedactatnee edie genmas 17 Nemichthys avocetta, Thread-eel oi cee cc ies ves te wasdertaweaesauenae de 17 Hippocampus hudsonius, Sea-horse.... 0.20... e ccc cence eee eens 17 Peprilus parw) War vests ss scced vic cys teatoieedas weary a cqeseasiaos alas ovanedicihle aaah’ 18 Lophius litulon, Anko or Fishing-frog. sc: cesds.evee sexcory eaves vexwen nes és 18 Epinephelus adscensionis, Rock-hind or Cabra Mora................2.2005- 20 Scales of Acanthoéssus bronnt acc vests caves vaneed dee Ned tee Sek WEA se dace 2 Gy Cloid) Scales cats oscia caecsives scraetoeint aan oie Wistay cue Guay ath avauealveshig Hata a MaNighelbieeeunes 22 Porichthys porosissimus, Singing-fish....... 0.0... e cece eee cee eee eens 23 Apomotis cyanellus, Blue-green Sunfish .......... 000 cece eee e eee e eee eee 27 Chiasmodon niger, Black Swallower. ....... 0... e cece eee e eee e eee ee eens 29 Jaws of a Parrot-fish, Sparisoma aurofrenatum. ..... 0. cece cece cence ees 30 Archosargus probatocephalus, Sheepshead... . 1.0.6... c cece cece e eee e eens 31 Campostoma anomalum, Stone-roller. . 0.0.0.6 60.0. c cece eee eens 33 Roccus lineatus, Striped. Bass: o2aaxicaccie eras ecnssaes dea eeunsaewe na vend 35 Roccus lineatus. Lateral View of Cranium ............ 6... cece cece eee 36 Roccus lineatus. Superior View of Cranium .......-..- 60sec eee eee eee eee 37 Roccus lineatus. Inferior View of Cranium...........---.+0+ eee eee eee eee 38 Roccus lineatus. Posterior View of Cranium. .............. cee eee eee eee 4o Roccus lineatus. Face-bones, Shoulder and Pelvic Girdles, and Hyoid Arch... 42 Lower Jaw of Ama calva, showing Gular Plate. ...............0 +. seen eee. 43 Roccus lineatus. Branchial Arches. .......... 006000 c esse dee e ete e eens 46 Pharyngeal Bone and Teeth of European Chub, Leuciscus cephalus........... 47 Upper Pharyngeals of Parrot-fish, Scarus strongylocephalus.........++++++++5 47 Lower Pharyngeal Teeth of Parrot-fish, Scarus strongylocephalus............. 47 Pharyngeals of Italian Parrot-fish, Spansoma cretense..... ++ +++ +++0eee sees 48 Roccus lineatus, Vertebral] Column and Appendages.......-.-.-+-+++220205- 48 Basal Bone of Dorsal Fin, Holoptychius leptopterus.... 0... 006 c eee eee ees 49 Inner View of Shoulder-girdle of Buffalo-fish, Icteobus bubalus.............-. 51 XVill List of Illustrations Pterophryne tumida, Sargassum-fish. . ... 2... 60-6 e cece eee eee Shoulder-girdle of Sebastolobus alascanus. .. 1.00.00 ccc cece cence eens Cranium of Sebastolobus clascanus:: 2. over ecg ce onde cane ge seen Lower Jaw and Palate of Sebastolobus alascanus.... 00.40.0000 creer nets Maxillary and Pre-maxillary of Sebastolobus alascanus .......-+0++ +0000 00> Part of Skeleton of Selene vomer.. cnc. vecaresies charge den ei aad ee Eas Hyostilic Skull of Chiloscyllium indicum, a Scyliorhinoid Shark... ....----- Skull of Heptranchias indicus, a Notidanoid Shark... .......------+-++55- Basal Bones of Pectoral Fin of Monkfish, Squatina...........--.+0000045. Pectoral Fin of Heterodontus PHU p Pipa. oe ak hee ee sg ew ne ES Fe Rw PéCtOral MimOf Lepr an chads CHa t CU Sic oreii sean pasado Stine we) EAMdlalah ances Shoulder-girdle of a Flounder, Paralichthys californicus................... Shoulder-girdle of a Toadfish, Batrachoides pacifict.................00..0. Shoulder-girdle of a Garfish, Tylosurus fodiator..................00..000. Shoulder-girdle of a Hake, Merluccius productus. 0.00.0... 0.000000 0 0000. Cladosclache filers, "REStOLEd 2. acco scats marcnde sy on HER ey Ese acto ioenom es ai Fold-like Pectoral and Ventral Fins of Cladoselache fyleri.... 0.0.0.0... Pectoral Fin of a Shark; Chilos¢yliutts os cesasna aes 6248 oS 4p Simbu dds Skull and Shoulder-girdle of Neoceratodus forsteri, showing archipterygium . . . A GONLNOGS SUES WON GG ss acs ai ih RA ass Scie lesb) Seah sansa Seaweed, Peek Avehcpeecooo doe Shoulder-pirdle of Acanthotssus. oc ucuywnkverds ged e weds st tuo diana ten Pectorals FanOh, PlCana Cait so cic iestacss eikceorine Gara econ Bae anh ee ORG GE Shoulder-girdle of Polypterus bichtrs <5... oe c205is sac canacad baevawswn an ogay Asotin: Of BO Biss sire Gas alana Aa paige AAs a ts pee Men eae onegeys oh mae Walaa vein + PlearacanthUs-decHenpens, <5 tcshavt dia wines Mowe seis eaenck Gna tia aelbiaee e Gie RR Ree os as Embryos of Heterodontus japonicus, a Cestraciont Shark.................. Polypterus congicus, a Crossopterygian Fish with External Gills. ........... Heterocercal Tail of Sturgeon, Acipenser slurio...........000 00000. eee Heterocercal Tail of Bowfin, Amia calva... 2... cece eee. Heterocercal Tail of Garpike, Lepisosteius osseus... 0.00000. Cory phenoides carapinus, showing Leptocercal Tail. ..................... Heterocercal Tail of Young Trout, Salmo fario... ..........0............ Isocercal Tail of Hake, Merluccius productus..........00000000000 00 -e Homocercal Tail of a Flounder, Paralichthys californicus ................ Gephyrocercal/ Tail; GiVWiolamiOlay. 2. Nw paves eis som loos ugiieede eyed ete ics ok olan eas Brad Of CAMUGTE NOUSEOS Dien sirndiin 088 ban BEERS ASSES dit Swick bh gems BLAINCOL A OL POPS CMM CGI ONIS rani. sen seciictce tie eae seem a ELMER EN geo PAGE List of Illustrations X1X Brain of a Perch, Perca flavescens. 2.0.0.0... ccc ccc cee cece ee eeeee gosh Petromyzon marinus unicolor. Head of Lake Lamprey, showing Pineal Body. rrz Chologaster cornutus, Dismal-swamp Fish................0.0.-000e cece eee 116 Typhlichthys subterraneus, Blind Cave-fish. ........0.00 0000 cece cece cece 116 Anableps dovit, Four-eyed Fish. ..........0.0.00. 00 cc ccc cece ee ccccceee 117 LPNOPS MUTI ON: cs on Darema scam ncnalee hawt Rite wee Pile weenie Meateaie et anans 118 Boleophthalmus chinensis, Pond-skipper...........0.02.0.0 0000 ccc ee cece e ee. 118 Lampetra wildert, Brook Lamprey.... 0.0.00... 00.000 c cece eee e eee e ee 120 Branchiostoma lanceolatum, European Lancelet.................0.0..0.0000. 120 Pseudupeneus maculatus, Goatfish. .......00.0000 000 ccc cece eee. 122 Xiphophorus hellert, Sword-tail Minnow. .................000 0-000 cece eee 124 Cymatogaster aggregatus, White Surf-fish, Viviparous, with Young............ 125 Goodea luitpoldt, a Viviparous Fish. ........5 00060 c ccc cue e eee eceeewaeees 126 Egg of Callorhynchus antarcticus, the Bottle-nosed Chimara................. 127 Egg of the Hagfish, Myxine limosa.... 2.20... eee aneneauaasa 127 Egg of Port Jackson Shark, Heterodontus philippi.. ..........0000..000.... 128 Development of Sea-bass, Centropristes striatus... 6.0... 00.0 cece cee eee ee 135 Ceninoprisies: sivratus; Séa-basSs.3 sv 2 4.009 248 dee ce etek soadacdae ways ded Gasies encvenianns 137 Atphias gladius, Young Swordfish. «<<: s8e es. cee seeWe roe PARE eames ennles 139 NUP HUGS: RIGA TUS, AS WOLGUS Dies. yous ach cain Mcita earns ab eee ss ean ao aie e pale Hex 139 Larva of the Sailfish, Isteophorus, Very Young. ................0....00000. 140 Larva of Brook Lamprey, Lampetra wilderi, before Transformation.......... 140 ANP UAUG Chrisy pe, COMMON Bl i2.9.c.cccie scp scsuele sdseuete tamed dudes woah ete we 140 Larva of Common Eel, Anguilla chrisypa, called Leptocephalus grassii....... 141 Larva of Sturgeon, Act penser Suirt 0. 55 cos ceca: tse-anars arts Cae ¥ a eu DEY Ae + I4I Waa OF Gnetodon sedentlartus: 2a cse tin e.cva diene aisron tad Rosacea AUeube GaAs neees anaes 142 Chetodon capistratus, Butterfly-fish. .......... 00. c cece eee ee ee 142 Mola mola, Very Early Larval Stage of Headfish, called Centaurus boops...... 143 Mola mola, Early Larval Stage called Molacanthus nummularis.............. 144 Mola mola, Advanced Larval Stagei. i0 2.2 wesstncncies nemes aoe Hee betes 144 Mola-mola. “Wexdtish, AN Gu lti2 sina i tedies cera Ah ayeeed Sa Aare ah emer 146 Albula vulpes. Transformation of Ladyfish from Larva to Young........... 147 Development of the Horsehead-fish, Selene vomer.... 1... .0. 00.00 e eee eens 148 Salans: yalocranius, MOCHSH sssisaie eck eee eautree soe eae notin 2 AER se ceelee s coelay 149 Dalita: pectoralis, Alaska Blackfish «5 24+22:sesee+ sans seresicesenevanedeats 149 Ophiocephalus barca, Snake-headed China-fish..........-..- 0.02... e esse 150 Carassius auratus, Monstrous Goldfish... . . Sits sau cian cease Mea cate SR eel her Say 51 Jaws of Nemichthys avocetta. 0... 0.10 cece eee eee nets 156 Cypselurus californicus, Flying-fish. .....-- 0-0-6600 e eect eee ees 157 Ammocrypta clara, Sand-darter. . ... 60.0060 eee eee eee 158 Fierasfer acus, Pearlfish, issuing from a Holocanthurian. .........-..-....... 159 Gobiomorus gronovii, Portuguese Man-of-war Fish. ..........-...-.....00. 160 Tide Pools:of Misakis21 ¢.cidsecweminsusd see tue pe alew yeaa awed ada. 161 Piychocheilus oregonensis, SQUa wih shite ce uieciactacanga sme euler reg ee as 162 Ptychocheilus grandis, Squawfish, Stranded as the Water Falls............... 164 XX List of Illustrations PAGE Larval Stages of Platophrys podas, a Flounder of the Mediterranean, showing Micration Of Bye ws sin 2h< ith ok Oasau nes rane a heres tae in Ne 174 Platophrys lunatus, the Wide-eyed Flounder. ..............00 00000 ees 175 Young Flounder Just Hatched, with Symmetrical Eyes...............---+055 175 Pseudopleuronectes americanus, Larval Flounder. ............2.5 202000000 es 176 Pseudopleuronectes americanus, Larval Flounder (more advanced stage).....-. 176 Face View of Recently-hatched Flounder.......-....-...2 2s eee e eee e ences 177 Sehilbtosus juriosus: Mad=Tomi 3 scsi c sack emacs Sorsuey se Herds R RE HANA Sn Oe 179 Emmydrichthys vulcanus, Black Nohu or Poison-fish ......--- 66.0.0 20000005 180 ‘Teuihis: bahionus, Brown Tantssicess.023 oredusaedk Wek es Pee Re hs seae 181 Stephanolepis hispidus, Common Filefish...... 2.0... 0+.¢008 00 see cece een es 182 TEI GOGOMINELCUREUSS © siciel oe beth) yu Shalsenctehayantecctne tard ess Gr sage aetennenieeranapae mae aadmoiina’ 183 Balistes carolinensis, the Trigger-fish. .. 6.00... 00. e cece eee eee eee ee 184 Narane-brasiuénsis, NUMbS: + wes snes eee nees same See eo eee oS 185 Lorpedosclectricus. Wlectwe Cat hShics-2 aiascchcon acrs tand 21a tate: boo eeine aie. Gdague sevd 2408 186 Asiroscopus gutiatus, Star-Cazers us we sdsien ed dudes eh wneay cea kee Disease oa de 187 Aithoprora. lucida, Headichtehshy ccm tcavantaomueeete vend ones eebee ae 188 Corynolophus reinhardti, showing Luminous Bulb.....................-0-- 188 TRING PLCNUS LUCEY ORS ca fata a. a teebeoz ayaritin st 2 fons a uN ef init igle BARA Ae Wa Re 189 AT ONY OD CLOCUST OLTEP Seman, 3m nsersuly Saysubies 8 2d ete 38 Iai RG Se trays cod doney nee ahos Igo Luminous Organs and Lateral Line of Midshipman, Porichthys notatus....... 192 Cross-section of Ventral Phosphorescent Organ of Midshipman, Porichthys NOUDUS So ers we pembinte B 5 an Ois dare ihn eS BONE ES A ats cd ung ans Gls Gd fpiedae eet ett 193 Section of Deeper Portion of Phosphorescent Organ, Porichthys notatus....... 194 Leplecheneis naucrates, Sucking-fish or Pegador. ...................0000000. 197 Caularchus m@andricus, Clingfish... 2.2.0.0... 6000 cece cece cece. 198 Polistotrema stoute, Hagish yas i.02glasie edaweaey ocauds we sent wv Alege din Bak edi 199 Pristis zysron, Indian Sawishis« 4.<.s82 se teie ths aug d sed ea pees wad ae va 200 Pristiophorus japonicus, Saw-shark .. 0.2... cece eee eee. 201 Skeletonvol Bice. Tso Luci Sics sds goo sls taig 5 qualsned 4 aepal Set sederg @nibtoreaes secs Lock 203 Skeleton of Red Rockfish, Sebastodes mintatus............................. 214 Skeleton of a Spiny-rayed Fish of the Tropics, Holacanthus ciliaris........... 214 Skeleton of the Cowfish, Lactophrys tricornis.............................. 215 Crystallias matsushime, Liparid. .. 2.0.6.0. e cece. 218 Sebastichthys maliger, Yellow-backed Rockfish...........0.0....0......... 218 Myoxocephalus scorpius, European Sculpin.............0.................. 219 Elemitripterus americanus, Sea-Traven. . . 66066 ce cence cecs... 220 Cycopterus lumpus, Lumpfish..... 2.2.0.0... 06 e ee ceee cece eee. 220 Psychrolutes paradoxus, Sleek Sculpin........... 0.00.00. 00.0 ccc cece. 221 Pallasina barbote; Agonoid-ish, 106.0000 kde ee suh oe hme eaten cnegesics. 221 Amblyopsis speleus, Blindfish of the Mammoth Cave.........0...00.00.00 2. 221 Lucifuga sublerranea, Blind Brotula. ..........-.. 06.60. B00 Fiypsy pops: rubicunda, Garibaldi. «10.4 sa094 ces eres sees cae once ae es: a Synanceia verrucosa, Gofu or Poison-fish..............00000.0.0....0002—~O es Alticns-saliens, Lizard-skipper. « s0a% needs alneas des asin a anene panies ae tes a List of Illustrations XX1 PAGE Etheostoma camurum, Blue-breasted Darter. ...........0.0.0 00 cee cece neues 231 Liuranus semicintus and Chlevastes colubrinus, Snake-eels...............-.. 233 Coral Reet:at cA plas sis aisytedsacs ywmuerene va ¥ ead. neues ave dei tee-na es Sapa bes 234 Rudarius ercodes, Japanese Filefish... 20.2.0... 0.0. e cece cece cee eee ens 241 Dewaodom Seto sus. (Gabel sh ysnnc sacha cecslaea te bata th aetery eaten a acd met leg Rous Gene BAA Dasyates:sabing, Stn @-tayseiwc? ven Ava oo te dav At San bo Hed aalh anc oa piweoart 246 Diplesion blennioides, Green-sided Darter. ...........0.00.00 000000 c cece 247 Hippocampus mohnikei, Japanese Sea-horse..........0 0.000 e eee ee 250 Archoplites interruptus, Sacramento Perch. ............0..0 0000000 e vee eee 258 Map of the Continents, Eocene Time.................. cece eee e eens 270 Caulophryne jordani, Deep-sea Fish of Gulf Stream... ..........00.0.00.000. 276 Exerpes asper, Fish of Rock-pools, Mexico. .........0. 0.00. e cee eee eee 276 DCMOCY SJ OS SUE Ss. ass pan er ror ie Soe Rrayteik Ses a aug alone Shas Patel Hila CIAL SENS A 279 Ictalurus punctatus, Channel Catfish . 2... ...0. 0... e eee eee eee ee ene 280 Drawing the Net on the Beach of Hilo, Hawaii........................005. 281 Semotilus atromaculatus, Horned Dace... ..........000. 0000 e cece eee eee eee 285 Leuciscus lineatus, Chub of the Great Basin. . ..............0 0s eee eee ee eens 287 Melletes: papilio; Butterfly Sculpiniesc033 ces yhaaewans needa voudeth ences cies 288 Scartichthys enosime, a Fish of the Rock-pools of the Sacred Island of Eno- SHIMa}. | APA yada ied oid aire ie a haus we dame amma see hes Rete creed eee ee 204 Halicheres bivittatus, the Slippery Dick....... 2.0.06... 60 c cece eee eee eee 207 PFLSLEATON MANTOHEM So accion 8 th MGA Aid fs Seana GD Meaenaeh ROR AME DANE bie Waist iue Teme ae 299 QOutletiof Lake Bonnevillé. ....secceda¢ coed poms Re WSs ews eee alee 303 Hypocritichthys analis, Silver Surf-fish.. 2.2.6.6. 0. 0c cece eee eee eee 309 Erimyzon sucetta, Creekfish or Chub-sucker... ......-- 60-0002 sees eee eee ees 315 Thaleichthys pretiosus, Eulachon or Ulchen.....-.-..-0-. 20-0 eee eee eee 320 Plecoglossus altivelis, the Japanese Ayu. ......-- 0-002 e eee eee eee eee ee eee 321 Coregonus clupeiformis, the Whitefish. ........... 00.0 eee cece ee eee eens 321 Mullus auratus, the Golden Surmullet.............20 0.000 e eee eee eee eee 322 Scomberomorus maculatus, the Spanish Mackerel: 22 cveatc du Senin Semmens wade 322 Lampris luna, the Opah or IMA OO TITISH oops entinceaten ic ngetn tudes aro tem heeewt almakn exe nee ate 323 Pomatomus saltatrix, the Bluefish. . ..........--. 0000s cece etree eee es 324 Centropomus undecimalis, the Robalo. .....-..-- +05 see eee e eee eee eee 324 Chetodipterus faber, the Spadefish. ...... 00-0... 00s e eee eter e nett eee ees 325 Micropterus dolomieu, the Small-mouthed Black Bass.....-.-..-.+-++-++++-- 325 Salvelinus fontinalis, the Speckled Trout .......--.-.. 05 sss erect eee eee 326 Salmo gairdnert, the Steelhead Trout.. .......- 0... eee erent eee eee 326 Salvelinus oquassa, the Rangeley Trout........-.- +0 +00 +00 seer eres eee ees 326 Salmo rivularis, the Steelhead Trout..... 0.0... 0-0-0 e eee eee eect eee eee 327 Salmo henshawi, the Tahoe Trout... . 0.2.06... e cece cere eters 327 Salvelinus malma, the Dolly Varden Trout......-.-. 0.020202 settee erences 327 Thymallus signifer, the Alaska Grayling. ....--++ ++ +++ ee +00 sere rere renee es 328 Esox lucius, the Pike... 00... 0600000 e teen ete t ete eee es 328 Pleurogrammus monopterygius, the Atka-fish........ ++... 000s see sree erences 328 Chirostoma humboldtianum, the Pescado blanco......-..-.-- 0.205 e reer teens 329 XX List of Illustrations PAGE Pseudupeneus maculatus, the Red Goatfish. ..........--0.+.0e reer eer 329 Pseudoscarus guacamaia, Great Parrot-fish. .............000000 50ers 330 Mugil cephalus, Striped Mullet. .......... 0.0. eee eee center n et tenses 330 Lutianus analis, Mutton=snapper ... 0 26 eevee cea aa eet ea CERT S ee te 331 Chipea harencus; Herring. a. = Fic. 38.—Cranium of Sebastolobus alascanus Gilbert. (After Starks.) Vv. Vomer. ALS. Alisphenoid. EO. = Exoccipital. N. Nasal. P. Parietal. EPO. Epiotic. E. Ethmoid. BA. Basisphenoid. SPO. Sphenotic. PF. Prefrontal. PRO. Prootic. PTO. Pterotic. FR. Frontal. BO. Barioccipital. PAS. Parasphenoid. SO. Supraoccipital. the less specialized of the bony fishes the pelvis is attached at a distance from the head among the muscles of the side, and free from the shoulder-girdle and other parts of the skeleton. The ventral fins are then said to be abdominal. When very close to the clavicle, but not connected with it, as in the mullet, the fin is still said to be abdominal or subabdominal. In the striped bass the pelvis is joined by ligament between the clavi- cles, near their tip. The ventral fins thus connected, as seen in most spiny-rayed fishes, are said to be thoracic. In certain forms the pelvis is thrown still farther forward and attached at the throat or even to the chin. When the ventral fins are thus inserted before the shoulder-girdle, they are said to be jugular. 54 The Skeleton of the Fish Most of the fishes with spines in the fins have thoracic ven- trals. In the fishes with jugular ventrals these fins have begun a process of degeneration by which the spines or soft rays or both are lost or atrophied. Degeneration.—By degeneration or degradation in biology is meant merely a reduction to a lower degree of complexity or specialization in structure. If in the process of development OP SOP Fra. 39.—Lower jaw and palate of Sebastolobus alascanus. (After Starks. ) PA. Palatine. AR. Articular. POP. Preopercle. MSPT. Mesopterygoid. AN. Angular, IOP. Interopercle. PT. Pterygoid. Q. Quadrate. SOP. Subopercle. MPT. Metapterygoid. SY. Symplectic. OP. Opercle. D. Dentary. HM. Hyomandibular. of the individual some particular organ loses its complexity it is said to be degenerate. If in the geological history of a type the same change takes place the same term is used. Degenera- tion in this sense is, hke specialization, a phase of adaptation. It does not imply disease, feebleness, or mutilation, or any ten- dency toward extinction. It is also necessary to distinguish clearly phases of primitive simplicity from the apparent sim- plicity resulting from degeneration. The Skeleton in Primitive Fishes.—To learn the names of bones we can deal most satisfactorily with the higher fishes, those in The Skeleton of the Fish ee which the bony framework has attained completion. But to understand the origin and relation of parts we must begin with the lowest types, tracing the different stages in the development of each part of the system. In the lancelets (Leptocardii), the verte- bral column consists simply of a gelatinous notochord extending from one end of the fish to the other, and pointed at both ends, no skull being developed. The notochord never shows traces of segmentation, although cartilaginous rods above it are thought to forecast apophyses. In these forms there is no trace of jaws, limbs, or ribs. In the embryo of the bony fish a similar notochord precedes the segmentation and ossification of the vertebral column. In Fia. 40.—Maxillary and premaxillary of Sebas- tolobus alascanus. M, maxillary; PM, pre- maxillary, most of the extinct types of fishes a notochord more or less Fic. 41.—Part of skeleton of Selene vomer (Linnzus). 56 The Skeleton of the Fish modified persisted through life, the vertebre being strung upon it spool fashion in various stages of development. In the Cyclo- stomi (lampreys and hagfishes) the limbs and lower jaw are still wanting, but a distinct skull is developed. The notochord is still present, but its anterior pointed end is wedged into the base of a cranial capsule, partly membranous, partly car- tilaginous. There is no trace of segmentation in the notochord itself in these or any other fishes, but neutral arches are fore- PII ADIADAIAS AIT] FPOPOY yy: Fig. 42.—Hyostylie skull of Chiloscyllium indicum, a Seyliorhinoid Shark. (After Parker and Haswell.) shadowed in a series of cartilages on each side of the spinal chord. The top of the head is protected by broad plates. Fig. 44. Fic. 43.—Skull of Heptranchias indicus (Gmelin), a notidanoid shark. (After Parker and Haswell.) Ita. 44.—Basal bones of pectoral fin of Monkfish, Squalina, (After Zittel.) Phere are ring-like cartilages supporting the mouth and other cartilages in connection with the tongue and gill structures The Skeleton of the Fish By The Skeleton of Sharks.—In the Elasmobranchs (sharks, rays, chimeras) the tissues surrounding the notochord are seg- mented and in most forms distinct vertebrae are developed. Each of these has a conical cavity before and behind, with a central canal through which the notochord is continued. The form and degree of ossification of these vertebree differ materially in the different groups. The skull in all these fishes is cartilaginous, forming a continuous undivided box containing the brain and lodging the organs of sense. To the skullin the shark is attached a suspensorium of one or two pieces supporting the mandible and the hyoid structures. In the chimera the mandible is articu- lated directly with the skull, the hyomandibular and quadrate Fic. 45. Fic. 46. Fic. 45.—Pectoral fin of Heterodontus philippi. (From nature.) Fic. 46.—Pectoral fin of Heptranchias indicus (Gmelin). (After Dean.) elements being fused with the cranium. The skullin such case is said to be autostylic, that is, with self-attached mandible. In the shark it is said to be hyostylic, the hyomandibular intervening. The upper jaw in the shark consists not of maxillary and premaxillary but of palatine elements, and the two halves of the lower jaw are representatives of Meckel’s cartilage, which is the cartilaginous centre of the dentary bone in the bony fishes. These jaw-bones in the higher fishes are in the nature of membrane bones, and in the sharks and their relatives all such bones are undeveloped. The hyoid structures are in the shark relatively simple, as are also the gill-arches, which vary in number. The vertical fins are supported by interneural and interhemal cartilages, to which the soft fin-rays are attached without articulation. The shoulder-girdle is made of a single cartilage, touching 58 The Skeleton of the Fish the back-bone at a distance behind the head. To this cartilage three smaller ones are attached, forming the base of the pectoral fin. These are called mesopterygium, proterygium, and meta- pterygium, the first named being in the middle and more distinctly basal. These three segments are subject to much varia- tion. Sometimes one of them is wanting; some- times two are grown to- gether. Behind these the fin-rays are attached. In most of the skates the shoulder-girdle is more closely connected with the anterior vertebre, which are more or less fused together. The pelvis, remote from the head, is formed, in the shark, of a’ single or paired cartilage with smaller elements at the base of the fin-rays. In the males a cartilaginous generative organ, known as the clasper, is attached to the pelvis and the ventral fins. In the Elasmobranchs the tail vertebree are progressively smaller backward. Ifa caudal fin is present, the last vertebrae are directed upward (heterocercal) and the greater part of the fin is below the axis. In other forms (sting-rays) the tail degenerates into a whip-like organ (lepto- cercal), often without fins. In certain primitive sharks (Ichthyo- tomi), as well as in the Dipnoi and Crossopterygii, the tail is diphycercal, the vertebree growing progressively smaller back- ward and not bent upward toward the tip. Fic. 47.—Shoulder-girdle of a Flounder, Para- lichthys californicus (Ayres). The Skeleton of the Fish 59 In the chimeras (Holocephali) the notochord persists and is The palate with the surrounded by a series of calcified rings. Fia. 48.—Shoulder-girdle of a Toadfish, Batrachoides pacifict (Ginther). suspensorium is coalesced with the skull, and the teeth are grown together into bony plates. The Archipterygium.——The Dipnoans, Crossopterygians, and Fie. 49.—Shoulder-girdle of a Garfish, Tylosurus fodiator (Jordan and Gilbert). Ganoids represent various phases of transition from the ancient cartilaginous types to the modern bony fishes. 60 The Skeleton of the Fish In the Ichthyotomous sharks, Dipnoans, and Crossoptery- gians the segments of the pectoral limb are arranged axially, or one beyond another. This type of fin has been called archipterygium by Gegenbaur, on the theory that it represents the condition shown on the first appearance of the pectoral fin. This theory is now seriously questioned, but it will be convenient to retain the name for the pectoral fin with segmented axis fringed on one or both sides by soft rays. Fic, 50.—Shoulder-girdle of a Hake, Merluccius productus (Ayres), The archipterygium of the Dipnoan genus Neoceratodus is thus described by Dr. Ginther (‘‘ Guide to the Study of Fishes,” p. 73): ‘‘ The pectoral limb is covered with small scales along the middle from the root to the extremity, and is surrounded bya rayed fringe similar to the rays of the vertical fins. A muscle spht into numerous fascicles extends all the length of the fin, which is flexible in every part and in every direction. The cartilaginous framework supporting it is joined to the scapular arch by a broad basal cartilage, generally single, sometimes The Skeleton of the Fish 61 showing traces of a triple division. Along the middle of the fin runs a jointed axis gradually becoming smaller and thinner towards the extremity. Each joint bears on each side a three-, two-, or one-jointed branch.” In the genus Lepidosiren, also a Dipnoan, the pectoral limb has the same axial structure, but is without fin-rays, although in the breeding season the posterior limb or ventral fin in the male is covered with a brush of fine filaments. This structure, according to Prof. J. G. Kerr,* is probably without definite function, but belongs to the ‘“‘category of modifications so often associated with the breeding season (cf. the newts’ crest) com- monly called ornamental, but which are perhaps more plausibly looked upon as expressions of the intense vital activity of the organisms correlated with its period of reproductive activity.” Professor Kerr, however, thinks it not unlikely that this brush of filaments with its rich blood-supply may serve in the function of respiration, a suggestion first made by Professor Lankester. * Philos. Trans., Lond., rgoo. CHAPTER V MORPHOLOGY OF THE FINS RIGIN of the Fins of Fishes.—-One of the most interest- ing problems in vertebrate morphology, and one of the most important from its wide-reaching relations, is that of the derivation of the fins of fishes. This resolves itself at once into two problems, the origin of the median fins, which appear in the lancelets, at the very bottom of the fish-like series, and the origin of the paired fins or limbs, which are much more complex, and which first appear with the primitive sharks. In this study the problem is to ascertain not what theoreti- cally should happen, but what, as a matter of fact, has happened in the early history of the fish-like groups. That these struc- tures, with the others in the fish body, have sprung from simple origins, growing more complex with the demands of varied conditions, and then at times again simple, through degenera- tion, there can be no doubt. It is also certain that each struc- ture must have had some element of usefulness in all its stages. In such studies we have, as Heckel has expressed it, “three ancestral documents, paleontology, morphology, and onto- geny ’’—the actual history as shown by fossil remains, the side- light derived from comparison of structures, and the evidence of the hereditary influences shown in the development of the individual. As to the first of these ancestral documents, the evidence of paleontology is conclusive where it is complete. But in very few cases are we sure of any series of details. The records of geology are like a book with half its leaves torn out, the other half confused, displaced, and blotted. Still each record actually existing represents genuine history, and in paleontology we must in time find our final court of appeal in all matters of biological origins. The evidence of comparative anatomy is most completely secured, but it is often indecisive as to relative age and primi- 62 Morphology of the Fins 63 tiveness of origin among structures. As to ontogeny, it is, of course, true that through heredity “the life-history of the indi- vidual is an epitome of the life-history of the race.” “ Onto- geny repeats phylogeny,” and phylogeny, or line of descent of organisms and structures, is what we are seeking. But here the repetition is never perfect, never nearly so perfect in fact as Heckel and his followers expected to find it. The demands of natural selection may lead to the lengthening, shortening, or distortion of phases of growth, just as they may modify adult conditions. The interpolation of non-ancestral stages is recognized in several groups. The conditions of the individual development may, therefore, furnish evidence in favor of cer- tain theories of origins, but they cannot alone furnish the abso- lute proof. In the process of development the median or vertical fins are doubtless older than the paired fins or limbs, whatever be the origin of the latter. They arise in a dermal keel which is developed in a web fitting and accentuating the undulatory motion of the body. In the embryo of the fish the continuous vertical fin from the head along the back and around the tail precedes any trace of the paired fins. In this elementary fin-fold slender supports, the rudiments of fin-rays, tend to appear at intervals. These are called by Ryder ray-hairs or actinotrichia. They are the prototype of fin-rays in the embryo fish, and doubtless similarly preceded the latter in geological time. In the development of fishes the caudal fin becomes more and more the seat of propulsion. The fin-rays are strengthened, their basal supports are more and more specialized, and the fin-fold ultimately divides into distinct fins, the longest rays developed where most needed. That the vertical fins, dorsal, anal, and caudal, have their origin in a median fold of the skin admits of no question. In the lowest forms which bear fins these structures are dermal folds, being supported by very feeble rays. Doubtless at first the vertical fins formed a continuous fold, extending around the tail, this fold ultimately broken, by atrophy of parts not needed, into distinct dorsal, anal, and caudal fins. In the lower fishes, as in the earlier sharks, there is an approach to this condition of primitive continuity, and in the embryos 64 Morphology of the Fins of almost all fishes the same condition occurs. Dr. John A. Ryder points out the fact that there are certain unexplained ex- ceptions to this rule. The sea-horse, pipefish, and other highly modified forms do not show this unbroken fold, and it is want- ing in the embryo of the top-minnow, Gambusia affinis. Never- theless the existence of a continuous vertical fold in the embryo is the rule, almost universal. The codfish with three dorsals. the Spanish mackerel with dorsal and anal finlets, the herring with one dorsal, the stickleback with a highly modified one, all show this character, and we may well regard it as a certain trait of the primitive fish. This fold springs from the ectoblast or external series of cells in the embryo. The fin-rays and bony supports of the fins spring from the mesoblast or middle series of cells, being thrust upward from the skeleton as supports for the fin-fold. Origin of the Paired Fins.—The question of the origin of the paired fins is much more difficult and is still far from settled, although many, perhaps the majority of recent writers favor the theory that these fins are parts of a once continuous lateral fold of skin, corresponding to the vertical fold which forms the dorsal, anal, and caudal. In this view the lateral fold, at first continuous, became soon atrophied in the middle, while at either end it is highly specialized, at first into an organ of direction, then into fan-shaped and later paddle-shaped organs of locomo- tion. According to another view, the paired fins originated from gill structures, originally both close behind the head, the ventral fin migrating backward with the progress of evolution of the species. Evidence of Paleontology.—If we had representations of all the early forms of fishes arranged in proper sequence, we could decide once for all, by evidence of paleontology, which form of fin appears first and what is the order of appearance. As to this, it is plain that we do not know the most primitive form of fin. Sharks of unknown character must have existed long before the earliest remains accessible to us. Hence the evidence of paleontology seems conflicting and uncertain. On the whole it lends most support to the fin-fold theory. In the later Devonian, a shark, Cladoselache fylert, is found in which the paired fins are lappet-shaped, so formed and placed as to suggest Morphology of the Fins 65 their origin from a continuous fold of skin. In this species the dorsal fins show much the same form. Other early sharks, con- stituting the order of Acanthodei, have fins somewhat similar, but each preceded by a stiff spine, which may be formed from coalescent rays. Long after these appears another type of sharks represented by Pleuracanthus and Cladodus, in which the pectoral fin is a Fic. 51.—Cladoselache fyleri (Newberry), restored. Upper Devonian of Ohio ; (After Dean.) jointed organ fringed with rays arranged serially in one or two rows. This form of fin has no resemblance to a fold of skin, but accords better with Gegenbaur’s theory that the pectoral limb was at first a modified gill-arch. In the Coal Measures are found also teeth of sharks (Orodontide) which bear a Fic. 52.—Fold-like pectoral and ventral fins of Cladoselache fyleri. (After Dean.) strong resemblance to still existing forms of the family of Heterodontide, which originates in the Permian. The existing Heterodontide have the usual specialized form of shark-fin, with three of the basal segments especially enlarged and placed side 66 Morphology of the Fins by side, the type seen in modern sharks. Whatever the primi- tive form of shark-fin, it may well be doubted whether any one of these three (Cladoselache, Pleuracanthus, or Heterodontus) actually represents it. The beginning is therefore unknown, though there is some evidence that Cladoselache is actually more nearly primitive than any of the others. As we shall see, the evidence of comparative anatomy may be consistent with either of the two chief theories, while that of ontogeny or em- bryology is apparently inconclusive, and that of paleontology is apparently most easily reconciled with the theory of the fin- fold. Development of the Paired Fins in the Embryo.—According to Dr. John A. Ryder (‘‘ Embryography of Osseous Fishes,’ 1882) “the paired fins in Teleostei arise locally, as short longitudinal folds, with perhaps a fewexceptions. The pectorals of Leprsosteus originate in the same way. Of the paired fins, the pectoral Fic. 53.—Pectoral fin of shark, Chiloscyllium. (After Parker and Haswell.) or anterior pair seems to be the first to be developed, the ventral or pelvic pair often not making its appearance until after the absorption of the yolk-sac has been completed, in other cases before that event, as in Salmo and in Gambusia. The pectoral fin undergoes less alteration of position during its evolution than the posterior pair.” In the codfish (Gadus callarias) the pectoral fin-fold ‘“ap- pears as a slight longitudinal elevation of the skin on either side of the body of the embryo a little way behind the auditory vesicles, and shortly after the tail of the embryo begins to bud out. At the very first it appears to be merely a dermal fold, and in some forms a layer of cells extends out underneath it from the sides of the body, but does not ascend into it. It Morphology of the Fins 67 begins to develop as a very low fold, hardly noticeable, and, as growth proceeds, its base does not expand antero-posteriorly, but tends rather to become narrowed, so that it has a peduncu- lated form. With the progress of this process the margin of the fin-fold also becomes thinner at its distal border, and at the basal part mesodermal cells make their appearance more notice- ably within the inner contour-line. The free border of the fin- fold grows out laterally and longitudinally, expanding the por- tion outside of the inner contour-line of the fin into a fan-shape. This distal thinner portion is at first without any evidence of rays; further than that there is a manifest tendency to a radial disposition of the histological elements of the fin.” The next point of interest is found in the change of position of the pectoral fin by a rotation on its base. This is associated with changes in the development of the fish itself. The ventral fin is also, in most fishes, a short horizontal fold and just above the preanal part of the median vertical fold which becomes anal, caudal, and dorsal. But in the top-minnow (Gambusia), of the order Haplomi, the ventral first appears as ‘‘a little papilla and not as a fold, where the body-walls join the hinder upper por- tion of the yolk-sac, a very little way in front of the vent.” ‘These two modes of origin,” observes Dr. Ryder, “ are therefore in striking contrast and well calculated to impress us with the protean character of the means at the disposal of Nature to achieve one and the same end.”’ Current Theories as to Origin of Paired Fins.—There are three chief theories as to the morphology and origin of the paired fins. The earliest is that of Dr. Karl Gegenbaur, supported by various workers among his students and colleagues. In his view the pectoral and ventral fins are derived from modifications of primitive gill-arches. According to this theory, the skeletal arrangements of the vertebrate limb are derived from modifica- tions of one primitive form, a structure made up of successive joints, with a series of fin-rays on one or both sides of it. To this structure Gegenbaur gives the name of archipterygium. It is found in the shark, Pleuwracanthus, in Cladodus, and in all the Dipnoan and Crossopterygian fishes, its primitive form being still retained in the Australian genus of Dipnoans, Neocera- todus. This biserial archipterygium with its limb-girdle is 68 Morphology of the Fins derived from a series of gill-rays attached to a branchial arch. The backward position of the ventral fin is due to a succession of migrations in the individual and in the species. As to this theory, Mr. J. Graham Kerr observes: “The Gegenbaur theory of the morphology of vertebrate limbs thus consists of two very distinct portions. The first, that the archipterygium is the ground-form from which all other forms of presently existing fin skeletons are derived, concerns us only indirectly, as we are dealing here only with the origin Fig. 54.—Skull and shoulder-girdle of Neoceratodus forsteri (Giinther), showing the archipterygium. of the limbs, i.e., their origin from other structures that were not limbs. “It is the second part of the view that we have to do with, that deriving the archipterygium, the skeleton of the primitive paired fin, from a series of gill-rays and involving the idea that the limb itself is derived from the septum between two gill-clefts. “This view is based on the skeletal structures within the fin. It rests upon (1) the assumption that the archipterygium is the primitive type of fin, and (2) the fact that amongst the Selachians is found a tendency for one branchial ray to become larger than the others, and, when this has happened, for the base of attachment of neighboring rays to show a tendency to migrate from the branchial arch on to the base of the larger or, as we may call it, primary ray; a condition coming about which, were the process to continue rather farther than it is known to do in actual fact, would obviously result in a struc- Morphology of the Fins 69 ture practically identical with the archipterygium. Gegenbaur suggests that the archipterygium actually has arisen in this way in phylogeny.” The fin-fold theory of Balfour, adopted by Dohrn, Weiders- heim, Thacher, Mivart, Ryder, Dean, Boulenger, and others, and Fic. 55.—Acanthoessus wardi (Egerton). Carboniferous. Family Acanthoesside. (After Woodward. ) now generally accepted by most morphologists as plausible, is this: that ‘‘The paired limbs are persisting and exaggerated portions of a fin-fold once continuous, which stretched along each side of the body and to which they bear an exactly similar Fic. 56.—Shoulder-girdle of Acan- Fic. 57.—Pectoral fin of Pleuracanthus. thoessus. (After Dean.) (After Dean.) phylogenetic relation as do the separate dorsal and anal fins to the once continuous median fin-fold.”’ “This view, in its modern form, was based by Balfour on his observation that in the embryos of certain Elasmobranchs 70 Morphology of the Fins the rudiments of the pectoral and pelvic fins are at a very early period connected together by a longitudinal ridge of thick- ened epiblast—of which indeed they are but exaggerations. In Balfour’s own words referring to these observations: ‘If the account just given of the development of the limb is an accu- rate record of what really takes place, it is not possible to deny that some light is thrown by it upon the first origin of the ver- tebrate limbs. The facts can only bear one interpretation, viz., that the limbs are the remnants of continuous lateral fins.’ “A similar view to that of Balfour was enunciated almost synchronously by Thacher and a little later by Mivart—in each case based on anatomical investigation of Selachians—mainly Fig. 58.—Shoulder-girdle of Polypterus bichir. Specimen from the White Nile. relating to the remarkable similarity of the skeletal arrange- ments in the paired and unpaired fins.” A third theory is suggested by Mr. J. Graham Kerr (Cam- bridge Philos. Trans., 1899), who has recently given a summary of the theories on this subject. Mr. Kerr agrees with Gegenbaur as to the primitive nature of the archipterygium, but believes that it is derived, not from the gill-septum, but from an external gill, Such a gill is well developed in the young of all the living sharks, Dipnoans and Crossopterygians, and in the latter types of fishes it has a form analogous to that of the archipterygium, although without bony or cartilaginous axis. We may now take up the evidence in regard to each of the different theories, using in part the language of Kerr, the para- Morphology of the Fins 71 graphs in quotation-marks being taken from his paper. We may first consider Balfour’s theory of the lateral fold. Balfour’s Theory of the Lateral Fold. —‘‘The evidence in regard to this view may be classed under three heads, as onto- genetic, comparative anatomical, and paleontological. The ultimate fact on which it was founded was Balfour's discovery that in certain Elasmobranch embryos, but especially in Tor- pedo (Narcobatis), the fin rudiments were, at an early stage, connected by a ridge of epiblast. I am not able to make out what were the other forms in which Balfour found this ridge, but subsequent research, in particular by Mollier, a supporter of the lateral-fold view, is to the effect that it does not occur in such ordinary sharks as Pristturus and Mustelus, while it is to be gathered from Balfour himself that it does not occur in Scyllium (Scyliorhinis). “Tt appears to me that the knowledge we have now that the longitudinal ridge is confined to the rays and absent in the less highly specialized sharks greatly diminishes its security as a basis on which to rest a theory. In the rays, in corre- lation with their peculiar mode of life, the paired fins have undergone (in secondary development) enormous ex- tension along the sides of the body, and their continu- ity in the embryo may well Be be a mere foreshadowing of Fic, 59.—Arm of a frog. this. “An apparently powerful support from the side of embry- ology came in Dohrn and Rabl’s discoveries that in Pristiurus all the interpterygial myotomes produce muscle-buds. This, however, was explained away by the Gegenbaur school as being merely evidence of the backward migration of the hind limb— successive myotomes being taken up and left behind again as the limb moved farther back. As either explanation seems an adequate one, I do not think we can lay stress upon this body of facts as supporting either one view or the other. The 72 Morphology of the Fins facts of the development of the skeleton cannot be said to support the fold view; according to it we should expect to find a series of metameric supporting rays produced which later on become fused at their bases. Instead of this we find a longi- tudinal bar of cartilage developing quite continuously, the rays forming as projections from its outer side. “The most important evidence for the fold view from the side of comparative anatomy is afforded by (1) the fact that the limb derives its nerve supply from a large number of spinal nerves, and (2) the extraordinary resemblance met with be- tween the skeletal arrangements of paired and unpaired fins. The believers in the branchial-arch hypothesis have disposed of the first of these in the same way as they did the occurrence of interpterygial myotomes, by looking on the nerves received from regions of the spinal cord anterior to the attachment of the limb as forming a kind of trail marking the backward migration of the limb. “The similarity in the skeleton is indeed most. striking, though its weight as evidence has been recently greatly dimin- ished by the knowledge that the apparently metameric segmen- tation of the skeletal and muscular tissues of the paired fins is quite secondary and does not at all agree with the meta- mery of the trunk. What resemblance there is may well be of a homoplastic character when we take into account the simi- larity in function of the median and unpaired fins, especially in such forms as Raja, where the anatomical resemblances are especially striking. There is a surprising dearth of paleonto- logical evidence in favor of this view.” The objection to the first view is its precarious foundation. Such lateral folds are found only in certain rays, in which they may be developed as a secondary modification in connection with the peculiar form of these fishes. Professor Kerr observes that this theory must be looked upon and judged: “‘ Just as any other view at the present time regarding the nature of the vertebrate limb, rather as a speculation, brilliant and suggestive though it be, than as a logically constructed theory of the now known facts. It is, I think, on this account allowable to apply to it a test of a character which is admittedly very apt to mislead, that of ‘common sense.’ Morphology of the Fins 73 “If there is any soundness in zoological speculation at all, I think it must be admitted that the more primitive vertebrates were creatures possessing a notochordal axial skeleton near the dorsal side, with the main nervous axis above it, the main viscera below it, and the great mass of muscle lying in myotomes along its sides. Now such a creature is well adapted to move- ments of the character of lateral flexure, and not at all for movements in the sagittal plane—which would be not only difficult to achieve, but would tend to alternately compress and extend its spinal cord and its viscera. Such a creature would swim through the water as does a Cyclostome, or a Lepidostren, or any other elongated vertebrate without special swimming organs. Swimming like this, specialization for more and more rapid movement would mean flattening of the tail region and ts extension into an at first not separately mobile median tail- fold. It is extremely difficult to my mind to suppose that a new purely swimming arrangement should have arisen involving up-and-down movement, and which, at its first beginnings, while useless as a swimming organ itself, must greatly detract from the efficiency of that which already existed.” Objections to Gegenbaur’s Theory.—We now return to the Gegenbaur view—that the limb is a modified gill-septum. “Resting on Gegenbaur’s discovery already mentioned, that the gill-rays in certain cases assume an arrangement showing great similarity to that of the skeletal elements of the archip- terygium, it has, so far as J am aware, up to the present time received no direct support whatever of a nature comparable with that found for the rival view in the fact that, in certain forms at all events, the limbs actually do arise in the individual in the way that the theory holds they did in phylogeny. No one has produced either a form in which a gill-septum becomes the limb during ontogeny, or the fossil remains of any form which shows an intermediate condition. “The portion of Gegenbaur’s view which asserts that the biserial archipterygial fin is of an extremely primitive charac- ter is supported by a large body of anatomical facts, and is rendered further probable by the great frequency with which fins apparently of this character occur amongst the oldest known fishes. On the lateral-fold view we should have to 74 Morphology of the Fins regard these as independently evolved, which would imply that fins of this type are of a very perfect character, and in that case we may be indeed surprised at their so complete disap- pearance in the more highly developed forms, which followed later on.” As to Gegenbaur’s theory it is urged that no form is known in which a gill-septum develops into a limb during the growth of the individual. The main thesis, according to Professor Kerr, “that the archipterygium was derived from gill-rays, is supported only by evidence of an indirect character. Gegen- baur in his very first sugestion of his theory pointed out, as a great difficulty in the way of its acceptance, the position of the Fia. 60,—Pleuracanthus decheni (Goldfuss). (After Dean.) limbs, especially of the pelvic limbs, in a position far removed from that of the branchial arches. This difficulty has been entirely removed by the brilliant work of Gegenbaur’s followers, who have shown from the facts of comparative anatomy and embryology that the limbs, and the hind limbs especially, ac- tually have undergone, and in ontogeny do undergo, an extensive backward migration. In some cases Braus has been able to find traces of this migration as far forward as a point just behind the branchial arches. Now, when we consider the numbers, the enthusiasm, and the ability of Gegenbaur’s dis- ciples, we cannot help being struck by the fact that the only evidence in favor of this derivation of the limbs has been that which tends to show that a migration of the limbs backwards has taken place from a region somewhere near the last bran- chial arch, and that they have failed utterly to discover any intermediate steps between gill-rays and archipterygial fin. And if for a moment we apply the test of common sense we cannot but be impressed by the improbability of the evolution of a gill-septum, which in all the lower forms of fishes is fixed Morphology of the Fins ae firmly in the body-wall, and beneath its surface, into an organ of locomotion. “May I express the hope that what I have said is sufficient to show in what a state of uncertainty our views are regarding the morphological nature of the paired fins, and upon what an Fie. 61.—Embryos of Heterodontus japonicus Maclay and Macleay, a Ces- traciont shark, showing the backward migration of the gill-arches and the forward movement of the pectoral fin. a, b, c, representing different stages of growth. (After Dean.) exceedingly slender basis rest both of the two views which at present hold the field?”’ As to the backward migration of the ventral fins, Dr. Bash- ford Dean has recently brought forward evidence from the embryo of a very ancient type of shark (Heterodontus japonicus) that this does not actually occur in that species. On the other 76 Morphology of the Fins hand, we have a forward migration of the pectoral fin, which gradually takes its place in advance of the hindmost gill-arches. The accompanying cut is from Dean’s paper, ‘‘ Biometric Evidence in the Problem of the Paired Limbs of the Verte- brates” (American Naturalist for November, 1902). Dean con- cludes that in Heterodontus ‘‘there is no evidence that there has ever been a migration of the fins in the Gegenbaurian sense.” “The gill region, at least in its outer part, shows no affinity during proportional growth with the neighboring region of the pectoral fin. In fact from an early stage onward, they are evi- dently growing in opposite directions.” Kerr’s Theory of Modified External Gills.—‘‘It is because I feel that in the present state of our knowledge neither of the two views I have mentioned has a claim to any higher rank than that of extremely suggestive speculations that I venture to say a few words for the third view, which is avowedly a mere speculation. ‘Before proceeding with it I should say that I assume the serial homology of fore and hind limbs to be beyond dispute. The great and deep-seated resemblances between them are such as to.my mind seem not to be adequately explicable except on this assumption. “In the Urodela (salamanders) the external gills are well- known structures—serially arranged projections from the body- wall near the upper ends of certain of the branchial arches. When one considers the ontogenetic development of these organs, from knob-like outgrowth from the outer face of the branchial arch, covered with ectoderm and possessing a meso- blastic core, and which frequently if not always appear before the branchial clefts are open, one cannot but conclude that they are morphologically projections of the outer skin and that they have nothing whatever to do with the gill-pouches of the gut-wall. Amongst the Urodela one such gill projects from each of the first three branchial arches. In Lepidosiren there is one on each of the branchial arches I-IV. In Polypterus and Calamotchthys (Erpetoichthys) there is one on the hyoid arch. Finally, in many Urodelan larvae we have present at the same time as the external gills a pair of curious structures called balancers. At an early stage of my work on Le pidosiren, Morphology of the Fins 77 while looking over other vertebrate embryos and larve for pur- poses of comparison, my attention was arrested by these struc- tures, and further examinations, by section or otherwise, convinced me that there were serial homologues of the external gills, situated on the mandibular arch. On then looking up the literature, I found that I was by no means first in this view. Rusconi had long ago noticed the resemblance, and in more recent times both Orr and Maurer had been led to the same conclusion as I had been. Three different observers having been inde- pendently led to exactly the same conclusions, we may, I think, fairly enough regard the view I have mentioned of the morphological nature of the balancers as probably a correct one. “Here, then, we have a series of homologous structures pro- jecting from each of the series of visceral arches. They crop up on the Crossopterygii, the Dipnoi, and the Urodela, 1.e., in three of the most archaic of the groups of Gnathostomata. But we may put it in another way. The groups in which they do not occur are those whose young possess a very large yolk-sac (or which are admittedly derived from such forms). Now wherever we have a large yolk-sac we have developed on its surface a rich network of blood-vessels for purposes of nutrition. But such a network must necessarily act as an extraordinarily efficient organ of respiration, and did we not know the facts we might venture to prophesy that in forms possessing it any other small skin-organ of respiration would tend to disappear. “No doubt these external gills are absent also in a few of the admittedly primitive forms such as, e.g., (Neo-) Ceratodus. But I would ask that in this connection one should bear in mind one of the marked characteristics of external gills—their great regenerative power. This involves their being extremely liable to injury and consequently a source of danger to their possessor. Their absence, therefore, in certain cases may well have been due to natural selection. On the other hand, the presence in so many lowly forms of these organs, the general close similarity in structure that runs through them in different forms, and the exact correspondence in their position and rela- tions to the body can, it seems to me, only be adequately ex- plained by looking on them as being homologous structures 78 Morphology of the Fins inherited from a common ancestor and consequently of great antiquity in the vertebrate stem.”’ As to the third theory, Professor Kerr suggests tentatively that the external gill may be the structure modified to form the paired limbs. Of the homology of fore and hind limbs and consequently of their like origin there can be no doubt. The general gill-structures have, according to Kerr, ‘the primary function of respiration. They are also, however, pro- vided with an elaborate muscular apparatus comprising elevators, depressors, and adductors, and larve possessing them may be seen every now and then to give them a sharp backward twitch They are thus potentially motor organs. In such a Urodele as Amblystoma their homologues on the mandibular arch are used as supporting structures against a solid substratum exactly as are the limbs of the young Lepidostren. “T have, therefore, to suggest that the more ancient Gna- thostomata possessed a series of potentially motor, potentially Fia. 62.—Polypterus congicus, a Crossopterygian fish from the Congo River. Young, with external gills. (After Boulenger.) supporting structures projecting from their visceral arches; it was inherently extremely probable that these should be made use of when actual supporting, and motor appendages had to be developed in connection with clambering about a solid sub- stratum. If this had been so, we should look upon the limb as a modified external gill; the limb-girdle, with Gegenbaur, as a modified branchial arch. “This theory of the vertebrate paired limb seems to me, I confess, to be a more plausible one on the face of it than either of the two which at present hold the field. If untrue, it is so dangerously plausible as to surely deserve more consideration than it appears to have had. One of the main differences be- tween it and the other two hypotheses is that, instead of deriving Morphology of the Fins 79 the swimming-fin from the walking and supporting limb, it goes the other way about. That this is the safer line to take seems to me to be shown by the consideration that a very small and rudimentary limb could only be of use if provided with a fixed point @appur. Also on this view, the pentadactyle limb and the swimming-fin would probably be evolved independently from a simple form of limb. This would evade the great diffi- culties which have beset those who have endeavored to estab- lish the homologies of the elements of the pentadactyle limb with those of any type of fully formed fin.”’ Uncertain Conclusions.—In conclusion we may say that the evi- dence of embryology in this matter is inadequate, though possibly favoring on the whole the fin-fold theory; that of morphology is inconclusive, and probably the final answer may be given by paleontology. If the records of the rocks were complete, they would be decisive. At present we have to decide which is the more primitive of two forms of pectoral fin actually known among fossils. That of Cladoselache is a low, horizontal fold of skin, with feeble rays, called by Cope ptychopterygium. That of Pleuracanthus is a jointed paddle-shaped appendage with a fringe of rays on either side. In the theory of Gegenbaur and Kerr Pleuracanthus must be, so far as the limbs are concerned, the form nearest the primitive limb-bearing vertebrate. In Balfour’s theory Cladoselache is nearest the primitive type from which the other and with it the archipterygium of later forms may be derived. Boulenger and others question even this, believing that the archipterygium in Pleuracanthus and other primitive sharks and that in Neoceratodus and its Dipnoan and Crossopterygian allies and ancestors have been derived independently, not the latter from the former. In this view there is no real homology between the archipterygium in the sharks possessing it and that in the Dipnoans and Crossopterygians. In the one theory the type of Pleuracanthus would be ancestral to the other sharks on the one hand, and to Crossopterygians and all higher vertebrates on the other. With the theory of the origin of the pectoral from a lateral fold, Pleuracanthus would be merely a curious specialized offshoot from the primitive sharks, without descend- ants and without special significance in phylogeny. 80 Morphology of the Fins As elements bearing on this decision we may note that the tapering unspecialized diphycercal tail of Pleuracanthus seems very primitive in comparison with the short heterocercal tail of Cladoselache. This evidence, perhaps deceptive, is balanced by the presence on the head of Pleuracanthus of a highly special- ized serrated spine, evidence of a far from primitive structure. Certainly neither the one genus nor the other actually repre- sents the primitive shark. But as Cladoselache appears in geological time, long before Plesuracanthus, Cladodus,. or any other shark with a jointed, archipterygial fin, the burden of proof, according to Dean, rests with the followers of Gegenbaur. If the remains found in the Ordovician at Cafion City referred to Crossopterygians are correctly interpreted, we must regard the shark ancestry as lost in pre-Silurian darkness, for in sharks of some sort the Crossopterygians apparently must find their remote ancestry. Forms of the Tail in Fishes——In the process of develop- ment the median or vertical fins are, as above stated, older than Fie. 63.—Heterocereal tail of Sturgeon, Acipenser sturio (Linneus). (After Zittel.) the paired fins or limbs, whatever be the origin of the latter. They arise in a dermal keel, its membranes fitting and accentuat- ing the undulatory motion of the body. In this elementary fin-fold slender supports (actinotrichia), the rudiments of fin-rays, appear at intervals. In those fins of most service in the movement of the fish, the fin-rays are strengthened, and their basal supports specialized. Dean calls attention to the fact that in fishes which swim, Morphology of the Fins 8] when adult, by an undulatory motion, the paired fins tend to disappear, as in the eel and in all eel-like fishes, as blennies and eel-pouts. The form of the tail at the base of the caudal fin varies in the different groups. In most primitive types, as in most embryonic fishes, the vertebrae grow smaller to the last (diphy- cercal). In others, also primitive, the end of the tail is directed upward, and the most of the caudal fin is below it. Such a tail is seen in most sharks, in the sturgeon, garpike, bowfin, and in the Ganoid fishes. It is known as heterocercal, and finally in ordinary fishes the tail becomes homocercal or fan- shaped, although usually some trace of the heterocercal condi- tion is traceable, gradually growing less with the process of development. , Since Professor Agassiz first recognized, in 1833, the dis- tinction between the heterocercal and homocercal tail, this matter has been the subject of elaborate investigation and a number of additional terms have been proposed, some of which are in common use. A detailed discussion of these is found in a paper by Dr. John A. Ryder ‘On the Origin of Heterocercy” in the Report of the U. S. Fish Commissioner for 1884. In this paper a dynamic or mechanical theory of the causes of change of form is set forth, parts of this having a hypothetical and somewhat uncertain basis. Dr. Ryder proposes the name archicercal to denote the cylin-. droidal worm-like caudal end of the larva of fishes and amphibi- ans before they acquire median fin-folds. The term lophocercalé - is proposed by Ryder for the form of caudal fin which consists of | a rayless fold of skin continuous with the skin of the tail, the | inner surfaces of this fold being more or less nearly in contact. To the same type of tail Dr. Jeffries Wyman in 1864 gave the name protocercal. This name was used for the tail of the larval ray when it acquires median fin-folds. The term implies, what cannot be far from true, that this form of tail is the first in the stages of evolution of the caudal fin. To the same type of tail Mr. Alexander Agassiz gave, in _ 1877, the name of leptocardial, on the supposition that it repre- sented the adult condition of the lancelet. In this creature, 82 _ Morphology of the Fins however, rudimentary basal rays are present, a condition differ- ing from that of the early embryos. * The diphycercal tail, as usually understood, is one in which the end of the vertebral column bears ‘‘not only hypural but also epural intermediary pieces which support rays.’, The term is used for the primitive type of tail in which the vertebre, lying horizontally, grow progressively smaller, as in Neocera- todus, Protopterus, and other Dipnoans and Crossopterygians. The term was first applied by McCoy to the tails of the Dipnoan genera Diplopterus and Gyroptychius, and for tails of this type it should be reserved. 4 The heterocercal tail is one in which the hindmost vertebre are bent upwards. The term is generally applied to those Fia. 64. Fic. 65 Fic. 64.—Heterocereal tail of Bowfin, Amia calva (Linneus). (After Zittel.) Fia. 65.—Heterocercal tail of Garpike, Lepisosteus osseus (Linnzus), fishes only in which this bending is considerable and is exter- nally evident, as in the sharks and Ganoids. The character disappears by degrees, changing sometimes to diphycercal or leptocercal by a process of degeneration, or in ordinary fishes becoming hhomocercal. Dr. Ryder uses the term heterocercal for all cases in which any upbending of the axis takes place, even though it involves the modification of but a single ver- tebra. With this definition, the tail of salmon, herne: and even of most bony fishes would be considered heterocercal, and most or all of these pass through a heterocercal stage in the course of development. The term is, however, usually restricted to those forms in which the curving of the axis is evident with- out dissection. Morphology of the Fins - 83 The homocercal tail is the fan-shaped or symmetrical tail common among the Teleosts, or bony fishes. In its process of development the individual tail is first archicercal, then lophocercal, then diphycercal, then heterocercal, and lastly homo- REE RESSESS a , Fic. 66.—Coryphenoides carapinus (Goode and Bean), showing leptocercal tail._ fishes in the rocks, although some forms of diplycercal tail may be produced by degeneration of the heterocerta) tail, as suggested ‘by Dr. Dollo and Dr. Boulenger, who divide di, liyceteal tails into primitive and secondary. . The peculiar tapering tail of the cod, the vertebre growing progressively smaller behind, is termed «socercal by Professor Cope. This form differs ‘tittle from diphycercal, except in its supposed derivation from the homocercal type. part. The cranium in fishes is relatively small, but the brain does not nearly fill its cavity, the space between the dura mater, which lines the skull-cavity, and the arachnoid membrane, which envelops the brain, being filled with a soft fluid containing a quantity of fat. The Brain of the Fish.—It is most convenient to examine the fish-brain, first in its higher stages of development, as seen in the sunfish, striped bass, or perch. As seen from above the brain of a typical fish seems to consist of five lobes, four of them in pairs, the fifth posterior to these and placed on the median line. The posterior lobe is the cerebellum, or metenceph- alon, and it rests on the medulla oblongata, the posterior portion of the brain, which is directly continuous with the spinal cord. In front of the cerebellum hes the largest pair of lobes, each of them hollow, the optic nerves being attached to the lower surface. These are known as the optic lobes, or mesencephalon In front of these lie the two lobes of the cerebrum, also called the hemispheres, or prosencephalon. These lobes are usually smaller than the optic lobes and solid. In some fishes they are crossed by a furrow, but are never corrugated as in the brain 109 110 The Nervous System of the higher animals. In front of the cerebrum lie the two small olfactory lobes, which receive the large olfactory nerve from the nostrils. From its lower surface is suspended the hy- pophysis or pituitary gland. In most of the bony fishes the structure of the brain does not differ materially from that seen in the perch. In the stur- Fic. 78, Fig. 78.—Brain of a Shark (Squatina squatina L.). (After Dean.) I. _ First cranial nerve (olfactory). V. Fifth cranial nerve. P. Prosencephalon (cerebrum). VII. Seventh cranial nerve. E. Epiphysis. V4. Fourth ventricle. T. Thalamencephalon. M. Mesencephalon (optic lobes). II. Second cranial nerve. MT. Metencephalon (medulla). IV. Fourth cranial nerve. EP. Epencephalon (cerebellum). Vig. 79.—Brain of Chimera monstrosa. (After Wilder per Dean.) Fic. 80.—Brain of Protopterus annectens. (After Burckhardt per Dean.) geon, however, the parts are more widely separated. In the Dipnoans the cerebral hemispheres are united, while the optic lobe and cerebellum are very small. In the sharks and rays the large cerebral hemispheres are usually coalescent into one, and the olfactory nerves dilate into large ganglia below the nostrils. The optic lobes are smaller than the hemispheres and also coa- lescent. The cerebellum is very large, and the surface of the The Nervous System Fi medulla oblongata is more or less modified or specialized. The brain of the shark is relatively more highly developed than that of the bony fishes, although in most other regards the latter are more distinctly specialized. The Pineal Organ.—Besides the structures noted in other fishes the epiphysis, or pineal organ, is largely developed in sharks, and traces of it are found in most or all of the higher vertebrates. In some of the lizards this epiphysis is largely developed, bear- Fic. 81. Fic. 81.—Brain of a Perch, Perca flavescens, (After Dean.) R. Olfactory lobe. II. Second cranial nerve. P. Cerebrum (prosencephalon). IV. Fourth cranial nerve. E. Epiphysis. V. Fifth cranial nerve. M. Optic lobes (mesencephalon). VII. Seventh cranial nerve. EP. Cerebellum (epencephalon), VIII. Eighth cranial nerve. ML. Medulla oblongata (metencephalon), IX. Ninth cranial nerve. I. First cranial nerve. X. Tenth cranial nerve. Fig. 82.—Petromyzon marinus unicolor (Dekay). Head of Lake Lamprey, showing pineal body. (After Gage.) ing at its tip a rudimentary eye. This leaves no doubt that in these forms it has an optic function. For this reason the struc- ture wherever found has been regarded as a rudimentary eye, and the ‘‘pineal eye’ has been called the ‘‘unpaired median eye of chordate’”’ animals. It has been supposed that this eye, once possessed by all vertebrate forms, has been gradually lost with the better de- 112 The Nervous System velopment of the paired eyes, being best preserved in reptiles as “an outcome of the life-habit which concealed the animal in sand or mud, and allowed the forehead surface alone to protrude, the median eye thus preserving its ancestral value in enabling the animal to look directly upward and backward.” This theory receives no support from the structures seen in the fishes. In none of the fishes is the epiphysis more than a nervous enlargement, and neither in fishes nor in amphibia is there the slightest suggestion of its connection with vision. It seems probable, as suggested by Hertwig and maintained by Dean that the original function of the pineal body was a nervous one and that its connection with or development into a median eye in lizards was a modification of a secondary character. On consideration of the evidence, Dr. Dean concludes that ‘‘the pineal structures of the true fishes do not tend to confirm the theory that the epiphysis of the ancestral vertebrates was con- nected with a median unpaired eye. It would appear, on the other hand, that both in their recent and fossil forms the epiphy- sis was connected in its median opening with the innervation of the sensory canals of the head. This view seems essentially confirmed by ontogeny. The fact that three successive pairs of epiphyseal outgrowths have been noted in the roof of the thala- mencephalon* appears distinctly adverse to the theory of a median eye.” ¢ The Brain of Primitive Fishes.—The brain of the hagfish differs widely from that of the higher fishes, and the homologies of the different parts are still uncertain. The different ganglia are all solid and are placed in pairs. It is thought that the cerebellum is wanting in these fishes, or represented by a narrow commissure (corpus restiforme) across the front of the medulla. In the lamprey the brain is more like that of the ordinary fish. In the lancelet there is no trace of brain, the band-like spinal cord tapering toward either end. The Spinal Cord.—The spinal cord extends from the brain to the tail, passing through the neural arches of the different ver- tebree when these are developed. In the higher fishes it is cylin- * The thalamencephalon or the interbrain is a name given to the region of the optic thalami, between the bases of the optic lobes and cerebrum } Fishes Recent and Fossil, p. 55. The Nervous System 113 drical and inelastic. In a few fishes (head-fish, trunk-fish) in which the posterior part of the body is shortened or degener- ate, the spinal cord 1s much shortened, and replaced behind by a structure called cauda equina. In the head-fish it has shrunk into ‘‘a short and conical appendage to the brain.’’ In the Cyclostomes and chimera the spinal cord is elastic and more or less flattened or band-like, at least posteriorly. The Nerves.—The nerves of the fish correspond in general in place and function with those of the higher animals. They are, however, fewer in number, both large nerve-trunks and smaller nerves being less developed than in higher forms. The olfactory nerves, or first pair, extend through the ethnoid bone to the nasal cavity, which is typically a blind sac with two roundish openings, but is subject to many variations. The optic nerves, or second pair, extend from the eye to the base of the optic lobes. In Cyclostomes these nerves run from each eye to the lobe of its own side. In the bony fishes, or Teleostei, each runs from the eye to the lobe of the opposite side. In the sharks, rays, chimeras, and Ganoids the two optic nerves are joined in a chiasma as in the higher vertebrates. Other nerves arising in the brain are the third pair, or ner- vus oculorum motorius, and the fourth pair, nervus trochlearts, both of which supply the muscles of the eye. The fifth pair, nervus trigeminus, and the seventh pair, nervus factalis, arise from the medulla oblongata and are very close together. Their various branches, sensory and motor, ramify among the mus- cles and sensory areas of the head. The sixth pair, nervus ab- ducens, passes also to muscles of the eye, and in sharks to the nictitating membrane or third eyelid. The eighth pair, nervus acousticus, leads to the ear. The ninth pair, glosso-pharyngeal, passes to the tongue and pharynx, and forms a ganglion connected with the sympathetic system. The tenth pair, nervus vagus, or pneumogastric nerve, arises from strong roots in the copus restiforme and the lower part of the medulla oblongata. Its nerves, motor and sensory, reach the muscles of the gill-cavity, heart, stomach, and air-bladder, as well as the muscular system and the skin. In fishes covered with bony plates the skin may be nearly or quite without sen- 114 The Nervous System sory nerves. The eleventh pair, nervus accessorius, and twelfth pair, nervus hypoglossus, are wanting in fishes. The spinal nerves are subject to some special modifications, but in the main correspond to similar structures in higher ver- tebrates. The anterior root of each nerve is without ganglionic enlargement and contains only motor elements. The posterior or dorsal root is sensory only and widens into a ganglionic swell- ing near the base. A sympathetic system corresponding to that in the higher vertebrates is found in all the Teleostei, or bony fishes, and in the body of sharks and rays in which it is not extended to the head. CHAPTER VIII THE ORGANS OF SENSE HE Organs of Smell.—The sense-organs of the fish cor- respond in general to those of the higher vertebrates. The sense of taste is, however, feeble or wanting, and aa of hearing is muffled and without power of acute discrimina- tion, if indeedit exists at all. According to Dr. Kingsley (Vert. Zool., p. 75), “recent experiments tend to show that in fishes the ears are without auditory functions and are solely organs of equilibration.” The sense of smell resides in the nostrils, which have no re- lation to the work of breathing. No fish breathes through its nostrils, and only in a few of the iowest forms (hagfishes) does the nostril pierce through the roof of the mouth. In the bony fishes the nostril is a single cavity, on either side, lined with delicate or fringed membrane, well provided with blood-vessels, and with nerves from the olfactory lobe. In most cases each nasal cavity has two external openings. These may be simple, or the rim of the nostril may be elevated, forming a papilla or even a long barbel. Either nostril may have a papilla or barbel, or the two may unite in one structure with two open- ings or with sieve-like openings, or in some degenerate types (Tro- pidichthys) with no obvious openings at all, the olfactory nerves spreading over the skin of a small papilla. The openings may be round, slit-like, pore-like, or may have various other forms. In certain families of bony fishes (Pomacentride, Cichlide, Hexagram- ide), there is but one opening to each nostril. In the sharks, rays, and chimeras there is also but one opening on either side and the nostril is large and highly specialized, with valvular flaps controlled by muscles which are said to enable them ‘‘to scent actively as well as to smell passively.” In the lancelet there is a single median organ supposed to 115 116 The Organs of Sense be a nostril, a small depression at the front of the head, covered by ciliated membrane. In the hagfish the single median nostril pierces the roof of the mouth, and is strengthened by carti- laginous rings, like those of the windpipe. In the lamprey the single median nostril leads to a blind sac. In the Barramunda (Neoceratodus) there are both external and internal nares, the former being situated just within the upper lip. In all other fishes there is a nasal sac on either side of the head. This has usually, but not always, two openings. There is little doubt that the sense of smell in fishes is rela- tively acute, and that the odor of their prey attracts them to Lo» Fig. 83.—Dismal Swamp Fish, Chologaster cornutus Agassiz. Supposed ancestor of Typhlichthys. Virginia, Fic. 84.—Blind Cavefish, Typhlichthys subterraneus Girard. Mammoth Cave, Kentucky. it. It is known that flesh, blood, or a decaying carcass will attract sharks, and other predatory fish are drawn in a similar manner. At the same time the strength of this function is yet to be tested by experiments. The Organs of Sight.—The eyes of fishes differ from those of the higher vertebrates mainly in the spherical form of the crys- talline lens. This extreme convexity is necessary because the lens itself is not very much denser than the fluid in which the fishes live. The eyes vary very much in size and somewhat in form and position. They are larger in fishes living at a mod- erate depth than in shore fishes or river fishes. At great depths, The Organs of Sense 117 as a mile or more, where all light is lost, they may become aborted or rudimentary, and may be covered by the skin. Often species with very large eyes, making the most of a little light or of light from their own luminous spots, will inhabit the same depths with fishes having very small eyes or eyes apparently useless for seeing, retained as vestigial structures through heredity. Fishes which live in caves become also blind, the structures showing every possible phase of degradation. The details of this gradual loss ot eyes, whether through reversed selection or hypothetically through inheritance of atrophy produced by disuse, have been given in a number of memoirs on the blind fishes of the Missis- sippi Valley by Dr. Carl H. Eigenmann. In some fishes the eye is raised on a short, fleshy stalk and can be moved about at the will of the fish. It is said that the vision of the pond-skipper, Periophihalmus, when hunting insects on the mud flats of Japan or India is “quite equal to that of a frog.” It is known also that trout possess keen Fic. 85.—Four-eyed Fish, Anableps dovii Gill, Tehuantepec, Mexico. eyesight, and that they show a marked preference for one sort or another of real or artificial fly. Nevertheless the vision of fishes in general is probably not very precise. They apparently notice motion rather than outline, changes rather than objects, while the extreme curvature of the crystalline lens would seem to render them all near-sighted. In the eyes of the fishes there is no lachrymal gland. True eyelids no fishes possess; the integuments of the head pass over the eye, becoming transparent as they cross the orbit. In some fishes part of this integument is thickened, covering the eye fully although still transparent. This forms the adipose eyelid char- acteristic of the mullet, mackerel, and lady-fish. Many of the sharks possess a distinct nictitating membrane or special eyelid, moved by a set of muscles. The iris in most fishes surrounds a 118 The Organs of Sense round pupil without much power of contraction. It is fre quently brightly colored, red, orange, black, blue, or green. In fishes, like rays or flounders, which lie on the bottom, a dark lobe covers the upper part of the pupil—a curtain to shut out light from above. The cornea is little convex, leaving small space for aqueous humor. In two genera of fishes, Anableps, Dialommus, the cornea is divided by a horizontal partition into Fic. 86.—Ipnops murrayi Ginther. two parts. This arrangement permits these fishes, which swim at the surface of the water, to see both in and out of the medium. Anableps, the four-eved fish, is a fresh-water fish of tropical America, which swims at the surface like a top-minnow, feeding on insects. Dialommus is a marine blenny from the Panama region, apparently of similar habit. In one genus of deep-sea fishes, [puops, the eyes are spread Fie. 87.—Pond-skipper, Boleophthalmus chinensis (Osbeck). Bay of Tokyo, Japan; from nature. K. Morita. (Eye-stalks shrunken in preservation.) out to cover the whole upper surface of the head, being modi- fied as luminous areas. Whether these fishes can see at all is not known. The position of the optic nerves is described in a previous chapter. In ordinary fishes there is one eye on each side of the head, but in the flounders, by a distortion of the cranium, both ap- The Organs of Sense 11g pear on the same side. This side is turned uppermost as the fish swims in the water or when it lies on the bottom. This distortion is a matter of development. The very young flounder swims with its broad axis vertical in the water, and it has one eye on either side. As soon as it rests on the bottom it begins to lean to one side. The lower eye changes its axis and by de- grees travels across the face of the fish, part of the bony inter- orbita] moving with it across to the other side. In some soles it is said to pass through the substance of the head, reappearing on the other side. In all species which the writer has examined the cranium is twisted, the eye moving with the bones; and the frontal bone is divided, a new orbit being formed by this division. In most northern flounders the eyes are on the right side in the adult, in tropical forms more frequently on the left, these distinctions corresponding with others in the structure of the fish. In the lowest of the fish-like forms, the lancelet, the eye is “simply a minute pigment-spot situated in the anterior wall of the ventricle at the anterior end of the central nervous system. In the hagfishes, which stand next highest in the series, the eye, still incomplete, is very small and hidden by the skin and mus- cles. This condition is very different from that of the blind fishes of the higher groups, in which the eye is lost through atrophy, because in life in caves or under rocks the function of seeing is no longer necessary. The Organs of Hearing.—The ear of the typical fish consists of the labyrinth only, including the vestibule and usually three semicircular canals, these dilating into sacs which contain one or more large, loose bones, the ear-stones or otoliths. In the lampreys there are two semicircular canals, in the hagfish but one. There is no external ear, no tympanum, and no Eustachian tube. The ear-sac on each side is lodged in the skull or at the base of the cranial cavity. It is externally surrounded by bone or cartilage, but sometimes it lies near a fontanelle or opening in the skull above. In some fishes it is brought into very close connection with the anterior end of the air-bladder. The latter organ it is thought may form part of the apparatus for hearing. The arrangement for this purpose is especially elaborate in the carp and the catfish families. In these fishes and their relatives 120 The Organs of Sense (called Ostartophyst) the two vestibules are joined in a median sac (stmus qmpar) in the substance of the basioccipital. This communicates with two cavities in the atlas, which again are supported by two small bones, these resting on a larger one Fic. 88.—Brook Lamprey, Lampetra wilder’ Jordan and Evermann, (After Gage.) Cayuga Lake. in connection with the front of the air-bladder. The system of bones is analogous to that found in the higher vertebrates, but it connects with the air-bladder, not with an external tympanum. The bones are not homologous with those of the ear of higher animals, being processes of the anterior vertebre. The tym- panic chain of higher vertebrates has been thought homologous with the suspensory of the mandible. The otoliths, commonly two in each labyrinth, are usually large, firm, calcareous bodies, with enamelled surface and peculiar Fie. 89.—European Lancelet, Branchiostoma lanceolatum (Pallas). (After Parker and Haswell.) grooves and markings. Each species has its own form of otolith, but they vary much in different groups of fishes. In the Elasmobranchs (sharks and rays) and in the Dipnoans the ear-sac is enclosed in the cartilaginous substance of the skull. There is a small canal extending to the surface.of the skull, ending sometimes in a minute foramen. The otoliths in these fishes are soft and chalk-like. The Organs of Sense I21 The lancelet shows no trace of an ear. In the cyclostomes, hagfishes, and lampreys it forms a capsule of relatively simple structure conspicuous in the prepared skeleton. The sense of hearing in fishes cannot be very acute, and is at the most confined to the perception of disturbances in the water. Most movements of the fish are governed by sight rather than by sound. It is in fact extremely doubtful whether fishes really hear at all, in a way comparable to the auditory sense in higher vertebrates. Recent experiments of Professor G. H. Parker on the killifish tend to show a moderate degree of auditory sense which grades into the sense of touch, the tubes of the lateral line assisting in both hearing and touch. While the killifish responds to a bass-viol string, there may be some fishes wholly deaf. Voices of Fishes.—Some fishes make distinct noises variously described as quivering, grunting, grating, or singing. The name grunt is applied to species of Hemulon and related genera, and fairly describes the sound these fishes make. The Spanish name ronco or roncador (grunter or snorer) is applied to several fishes, both scizenoid and hemuloid. The noise made by these fishes may be produced by forcing air from part to part of the com- plex air-bladder, or it may be due to grating one on another of the large pharyngeals. The grating sounds arise, no doubt, from the pharyngeals, while the quivering or singing sounds arise in the air-bladder. The midshipman, Porichthys notatus, is often called singing fish, from a peculiar sound it emits. These sounds have not yet been carefully investigated. The Sense of Taste.—It is not certain that fishes possess a sense of taste, and it is attributed to them only through their homology with the higher animals. The tongue is without deli- cate membranes or power of motion. In some fishes certain parts of the palate or pharyngeal region are well supplied with nerves, but no direct evidence exists that these have a function of discrimination among foods. Fishes swallow their food very rapidly, often whole, and mastication, when it takes place, is a crushing or cutting process, not one likely to be affected by the taste of the food. The Sense of Touch.—The sense of touch is better developed among fishes. Most of them flee from contact with actively 122 The Organs of Sense moving objects. Many fishes use sensitive structures as a means of exploring the bottom or of feeling their way to their food. The barbel or fleshy filament wherever developed is an organ of touch. In some fishes, barbels are outgrowths from the nostrils. In the catfish the principal barbel grows from the rudimentary maxillary bone. In the horned dace and gudgeon the little barbel is attached to the maxillary. In other fishes barbels grow from the skin of the chin or snout. In Ke Fic. 90.—Goat-fish, Pseudupeneus maculatus (Bloch). Woods Hole. the goatfish and surmullet the two chin barbels are highly specialized. In Polymixia the chin barbels are modified branchiostegals. Inthe codfish the single beard is little developed. In the gurnards and related forms the lower rays of the pectoral are separate and barbel-like. Detached rays of this sort are found in the thread-fins (Polynemid@), the gurnards (Triglide), and in various other fishes. Barbels or fleshy flaps are often developed over the eyes and sometimes on the scales or the fins. The structure of the lateral line and its probable relation as a sense-organ is discussed on page 23. It is probable that it is associated with sense of touch, and hearing as well, the internal ear being originally ‘‘a modified part of the lateral-line system,” as shown by Parker,* who calls the skin the lateral line and the ear “three generations of sense-organs.”’ * See Parker, on the sense of hearing in fishes, American Naturalist for March, 1903. The Organs of Sense t22 The sense of pain is very feeble among fishes. A trout has been known to bite at its own eye placed on a hook, and similar insensibility has been noted in the pike and other fishes. “The Greenland shark, when feeding on the carcass of a whale, allows itself to be repeatedly stabbed in the head without abandoning its prey.” (GUNTHER.) CHAPTER IX THE ORGANS OF REPRODUCTION IHE Germ-cells.—In most fishes the germ-cells are pro- duced in large sacs, ovaries or testes, arranged sym- G+) metrically one on either side of the posterior part of the Guia cavity. The sexes are generally but not always similar externally, and may be distinguished on dissection by the difference between the sperm-cells and the ova. The ovary Fig. 91.—Sword-tail Teen male, Xiphophorus hellert Heckel. The anal fin modified as an intromittent organ. Vera Cruz. with its eggs is more yellow in color and the contained cells appear granular. The testes are whitish or pinkish, their secre- tion milk-like, and to the naked eye not granular. In a very few cases both organs have been found in the same fish, as in Serranus, which is sometimes truly hermaphrodite. All fishes, however, seem to be normally dicecious, the two sexes in different individuals. Usually there are no external genital organs, but in some species a papilla or tube is developed at the end of the urogenital sinus. This may exist in the breeding season only, as in the fresh-water lampreys, or it may persist through life as in some gobies. In the Elasmobranchs, carti- laginous claspers, attached to the ventral fins in the male, serve as a conduit for the sperm-cells. 124 The Organs of Reproduction 126 The Eggs of Fishes.—The great majority of fishes are ovipa- rous, the eggs being fertilized after deposition. »The eggs are laid in gravel or sand or other places suitable for the species, and the milt containing the sperm-cells of the male is discharged over or among them in the water. A very small quantity of the sperm- fluid may impregnate a large number of eggs. But one sperm- cell can enter a particular egg. In a number of families the species are ovoviviparous, the eggs being hatched in the ovary or in a dilated part of the oviduct, the latter resembling a real uterus. In some sharks there is a structure analogous to Fic. 92.—White Surf-fish, viviparous, with young, Cymatogaster aggregatus Gibbons. San Francisco. the placenta of higher animals, but not of the same structure ororigin. Inthe case of viviparous fishes actual copulation takes place and there is usually a modification of some organ to effect transfer of the sperm-cells. This is the purpose of the sword- shaped anal fin in many top-minnows (Pecilide), the fin itself being placed in advance of its usual position. In the surf-fishes (Embiotocide) the structure of part of the anal fin is modified, although it is not used as an intromittent organ. In the Elas- mobranchs, as already stated, large organs of cartilage (claspers) are developed from the ventral fins. In some viviparous fishes, as in the rockfishes (Sebastodes) and rosefishes (Sebastes), the young are very minute at birth. gzl Coy 1997V) mpupowg SUR p “ootNoyy forenozyrgq oye'y Woy ysy snomedtata Wo Gauqorpuleys) ipjodpn) vapooy— eG “OT The Organs of Reproduction Loy In others, as the surf-fishes (Embiotocide), they are relatively large and few in number. In the viviparous sharks, which con- stitute the majority of the species of living sharks, the young are large at birth and prepared to take care of themselves. The eggs of fishes vary very much in size and form. In Ws 4 aad Ma ay Uf Chip / \ ae} Fic. 94.—Egg of Callorhynchus antarcticus, the Bottle-nosed Chimera. (After Parker and Haswell.) those sharks and rays which lay eggs the ova are deposited in a horny egg-case, in color and texture suggesting the kelp in which they arelaid. The eggs of the bull-head sharks (Heterodon- tus) are spirally twisted, those of the cat-sharks (Scyliorhinidc) are quadrate with long filaments at the angles. Those of rays are wheelbarrow-shaped with four ‘‘handles.” One egg-case NR J ota NOT eS — Fic. 95.—Egg of the Hagfish, Myzine limosa Girard, showing threads for attach- ment. (After Dean.) of a ray may sometimes contain several eggs and develop several young. The eggs of lancelets are small, but those of the hagfishes are large, ovate, with fibres at each side, each with a triple hook at tip. The chimera has also large egg-cases, oblong in form. In the higher fishes the eggs are spherical, large or small according to the species, and varying in the firmness of their 128 The Organs of Reproduction outer walls. All contain food-yolk from which the embryo in its earlier stages is fed. The eggs of the eel (Anguilla) are micro- scopic. According to Gunther 25,000 eggs have been counted in the herring, 155,000 in the lumpfish, 3,500,000 in the halibut, 635,200 in the sturgeon, and 9,344,000 in the cod. Smaller numbers are found in fishes with large ova. The red salmon has about 3500 eggs, the king salmon about 5200. Where an oviduct is present the eggs are often poured out in glutinous masses, as in the bass. When, as in the salmon, there is no oviduct, the eggs lie separate and do not cohere together. It is only with the latter class of fishes, those in which the eggs remain distinct, that artificial impregnation and — hatching is practicable. In this re- Fic. 96.—Egg of Port Jackson Shark, Heterodontus philippi gard the value of the salmon and (Lacépede). (After Parker and trout is predominant. In some fishes, Haswell.) especially those of elongate form, as the needle-fish (Tylosurus), the ovary of but one side is developed. Protection of the Young.—In most fishes the parents take no care of their eggs or young. In some catfishes (Platystacus) the eggs adhere to the under surface of the female. In a kind of pipefish (Solenostomus), a large pouch for retention of the eggs is formed on the belly of the female. In the sea-horses and pipefishes a pouch is formed in the skin, usually underneath the tail of the male. Into this the eggs are thrust, and here the young fishes hatch out, remaining until large enough to take care of themselves. In certain sea catfishes (Galeichthys, Cono- rhynchos) the male carries the eggs in his mouth, thus protecting them from the attacks of other fishes. In numerous cases the male constructs a rough nest, which he defends against all in- truders, against the female as well as against outside enemies. The nest-building habit is especially developed in the stickle- The Organs of Reproduction 129 backs (Gasterosteide), a group in which the male fish, though a pygmy in size, is very fierce in disposition. In a minnow of Europe (Riiodeus amarus) the female is said to deposit her eggs within the shells of river mussels. Sexual Modification.—In the relatively few cases in which the sexes are unlike the male is usually the brighter in color and with more highly developed fins. Blue, red, black, and silvery-white pigment are especially characteristic of the male, the olivaceous and mottled coloration of the female. Sometimes the male has a larger mouth, or better developed crests, barbels, or other appendages. In some species the pattern of coloration in the two sexes is essentially differént. In various species the male develops peculiar structures not found in the female, and often without any visible purpose. In the chimeera a peculiar cartilaginous hook armed with a brush of enamelled teeth at the tip is developed on the forehead in the male only. In the skates or true rays (Raja) the pectoral fin has near its edge two rows of stout incurved spines. These the female lacks. In the breeding season, among certain fishes, the male sometimes becomes much brighter by the accumulation of bright red or blue pigment accompanied by black or white pig- ment cells. This is especially true in the minnows (Notropis), the darters (Etheostoma), and other fresh-water species which spawn in the brooks of northern regions in the spring. Inthe minnows and suckers horny excrescences are also developed on head, body, or fins, to be lost after the deposition of the spawn. In the salmon, especially those of the Pacific, the adult male becomes greatly distorted in the spawning season, the jaws and teeth being greatly elongated and hooked or twisted so that the fish cannot shut its mouth. The Atlantic salmon and the trout show also some elongation of the jaws, but not to the same extent. In those fishes which pair the relation seems not to be per- manent, nor is there anything to be called personal affection among them so far as the writer has noticed. There is no evidence that the bright colors or nuptial adorn- ments of the males are enhanced by sexual selection. In most species the males deposit the sperm-cells in spawning-grounds 130 The Organs of Reproduction without much reference to the preference of the females. In general the brightest colors are not found among viviparous fishes. None of the groups in which the males are showily colored, while the females are plain, belong to this class. The brightest colors are found on the individuals most mature or having greatest vitality. CHAPTER X EMBRYOLOGY AND GROWTH OF FISHES EGMENTATION of the Egg.—The egg of the fish de- velops only after fertilization (amphimixis). This process is the union of its nuclear substance with that of the sperm-cell from the male, each cell carrying. its equal share in the function of heredity. When this process takes place the egg is ready to begin its segmentation. The eggs of all fishes are single cells containing more or less food- yolk. The presence of this food-yolk affects the manner of segmentation in general, those eggs having the least amount of food-yolk developing most typically. The simplest of all fish- like vertebrates, the lancelet (Branchiostoma) has very small eggs, and in their early development it passes through stages that are typical for all many-celled animals. The first stage in development is the simple splitting of the egg into two halves. These two daughter cells next divide so that there are four cells; each of these divides, and this division is repeated until a great number of cells is produced. The phenomenon of repeated di- vision of the germ-cell is called cleavage, and this cleavage is the first stage of development in the case of all many-celled animals, Instead of forming a solid mass the cells arrange themselves in such a way as to form a hollow ball, the wall being a layer one cell thick. The included cavity is called the segmentation cavity, and the whole structure is known as a blastula. This stage also is common to all the many-celled animals. The next stage is the conversion of the blastula into a double- walled cup, known as a gastrula by the pushing in of one side. All the cells of the blastula are very small, but those on one side are somewhat larger than those of the other, ‘ and here the wall first flattens and then bends in until finally the larger cells come into contact with the smaller and the segmentation cavity is entirely obliterated. There is now 131 T32 Embryology and Growth of Fishes an inner layer of cells and an outer layer, the inner layer being known as the endoblast and the outer as the ectoblast. The cavity of the cup thus formed is the archenteron and gives tise primarily to the alimentary canal. This third well-marked stage is called the gastrula stage, and it is thought to occur either typically or in some modified form in the development of all metazoa, or many-celled animals. In the lampreys, the Ganoids, and the Dipnoans the eggs contain a much greater quantity of yolk than those of the lancelet, but the segmenta- tion resembles that of the lancelet in that it 1s complete; that is, the whole mass of the egg divides into cells. There is a great difference, however, in the size of the cells, those at the upper pole being much smaller than those at the lower. In Petromyzon and the Dipnoans blastula and gastrula stages result, which, though differing in some particulars from the corresponding stages of the lancelet, may yet readily be compared with them. In the hagfishes, sharks, rays, chimzeras, and most bony fishes there is a large quantity of yolk, and the protoplasm, instead of being distributed evenly throughout the egg, is for the most part ac- cumulated upon one side, the nucleus being within this mass of protoplasm. When the food substance or yolk is consumed and the little fish is able to shift for itself, it leaves the egg-envelopes and is said to be hatched. The figures on page 135 show some of the stages by which cells are multiplied and ultimately grouped together to form the little fish. Post-embryonic Development.—In all the fishes the develop- ment of the embryo goes on within the egg long after the gastrula stage is passed, and until the embryo becomes a complex body, composed of many differing tissues and organs. Almost all the development may take place within the egg, so that when the young animal hatches there is necessary little more than a rapid growth and increase of size to make it a fully developed mature animal. This is the case with most fishes: a little fish just hatched has most of the tissues and organs of a full-grown fish, and is simply a small fish. But in the case of some fishes the young hatches from the egg before it has reached such an ad- vanced state of development, and the young looks very different from its parent. It must yet undergo considerable change before it reaches the structural condition of a fully developed Embryology and Growth of Fishes 134 and fully grown fish. Thus the development of most fishes is almost wholly embryonic development—that is, development within the egg or in the body of the mother—while the develop- ment of some of them is to a considerable degree post-embry- onic or larval development. There is no important difference between embryonic and post-embryonic development. The de- velopment is continuous from egg-cell to mature animal and, whether inside or outside of an egg, it goes on with a degree of regularity. While certain fishes are subject to a sort of meta- morphosis, the nature of this change is in no way to be com- pared with the change in insects. which undergo a complete metamorphosis. In the insects all the organs of the body are broken down and rebuilt in the process of change. In all fishes a structure once formed maintains a more nearly continuous integrity although often considerably altered in form. General Laws of Development.—The general law of develop- ment may be briefly stated as follows: All many-celled animals begin life as a single cell, the fertilized egg-cell; each animal goes through a certain orderly series of developmental changes which, accompanied by growth, leads the animal to change from single-cell to many-celled, complex form characteristic of the species to which the animal belongs; this development is from simple to complex structural condition; the development is the same for all individuals of one species. While all animals begin development similarly, the course of development in the dif- ferent groups soon diverges, the divergence being of the nature of a branching, like that shown in the growth of atree. In the free tips of the smallest branches we have represented the various species of animals in their fully developed condition, all standing clearly apart from each other. But in tracing back the development of any kind of animal we soon come to a point where it very much resembles or becomes apparently identical with some other kind of animal, and going farther back we find it resembling other animals in their young condition, and so on until we come to that first stage of development, that trunk stage where all animals are structurally alike. Any ani- mal at any stage in its existence differs absolutely from any other kind of animal, in this respect: it can develop into only its own kind. There is something inherent in each develop- 134 Embryology and Growth of Fishes ing animal that gives it an identity of its own. Although in its young stages it may be indistinguishable from some other species of animal in its young stages, it is sure to come out, when fully developed, an individual of the same kind as its parents were or are. The young fish and the young salamander may be alike to all appearance, but one embryo is sure to develop into a fish, and the other into a salamander. This certainty of an embryo to become an individual of a certain kind is called the law of heredity. Viewed in the light of development, there must be as great a difference between one egg and another as between one animal and another, for the greater difference is included in the less. The Significance of Facts of Development.—The significance of the process of development in any species is yet far from com- pletely understood. It is believed that many of the various stages in the development of an animal correspond to or repeat the structural condition of the animal’s ancestors. Naturalists believe that all animals having a notochord at any stage in their existence are related to each other through being descended from a common ancestor, the first or oldest chordate or back- boned animal. In fact it is because all these chordate animals— the lancelets, lampreys, fishes, batrachians, the reptiles, the birds, and the mammals—have descended from a common ancestor that they all develop a notochord, and those most highly organized re- place this by a complete back-bone. It is believed that the de- scendants of the first back-boned animal have, in the course of many generations, branched off little by little from the original type until there came to exist very real and obvious differences among the back-boned animals—differences which among the liv- ing back-boned animals are familiar to all of us. The course of development of an individual animal is believed to be a verv rapid and evidently much condensed and changed recapitula- tion of the history which the species or kind of animal to which the developing individual belongs has passed through in the course of its descent through a long series of gradually changing ancestors. If this is true, then we can readily understand why the fish and the salamander and the tortoise and bird and rabbit are all alike in their earlier stages of development, and gradually Embryology and Growth of Fishes 135 come to differ more and more as they pass through later and later developmental stages. Development of the Bony Fishes.* The mode of develop- ment of bony fishes differs in many and apparently important regards from that of their nearest kindred, the Ganoids. In their eggs a large amount of yolk is present, and its relations to the embryo have become widely specialized. As a rule, the egg of a Teleost is small, perfectly spherical, and enclosed in delicate but greatly distended membranes. The germ disc is especially small, appearing on the surface as an almost trans- parent fleck. Among the fishes whose eggs float at the sur- Fic. 97.—Development of Sea-bass, Centropristes striatus (Linneus). a, egg prior to germination; b, germ-disk after first cleavage; c, germ-disk after third cleavage; d, embryo just before hatching. (After H. V. Wilson.) face during development, as of many pelagic Teleosts, e.g., the sea-bass, Centropristes striatus, the yolk is lighter in specific gravity than the germ; it is of fluid-like consistency, almost transparent. In the yolk at the upper pole of the egg an oil globule usually occurs; this serves to lighten the relative weight of the entire egg, and from its position must aid in keeping this pole of the egg uppermost. In the early segmentation of the germ the first cleavage plane is established, and the nuclear divisions have taken place for the second; in the latter the third cleavage has been com- pleted. As in other fishes these cleavages are vertical, the third parallel to the first. A segmentation cavity occurs as a central space between the blastomeres, as it does in the sturgeon and garpike. In stages of late segmentation the segmentation cavity is * This account of the normal development of the Teleost fishes is condensed from Dr. Dean’s ‘‘ Fishes Living and Fossil,’’ in which work the details of growth in the Teleost are contrasted with those of other types of fishes. 136 Embryology and Growth of Fishes greatly flattened, but extends to the marginal cells of the germ- disk; its roof consists of two tiers of blastomeres, its floor of a thin film of the unsegmented substance of the germ; the mar- ginal blastomeres are continuous with both roof and floor of the cavity, and are produced into a thin film which passes downward, around the sides of the yolk. Later the segmenta- tion cavity is still further flattened; its roof is now a dome- shaped mass of blastomeres; the marginal cells have multiplied, and their nuclei are seen in the layer of the germ, below the plane of the segmentation cavity. These are seen in the sur- face view of the marginal cells of this stage; they are separated by cell boundaries only at the sides; below they are continuous in the superficial down-reaching layer of the germ. The mar- ginal cells shortly lose all traces of having been separate; their nuclei, by continued division, spread into the layer of germ flooring the segmentation cavity, and into the delicate film of germ which now surrounds the entire yolk. Thus is formed the periblast of the Teleost development, which from this point on- ward is to separate the embryo from the yolk; it is clearly the specialized inner part of the germ, which, becoming fluid- like, loses its cell-walls, although retaining and multiplying its nuclei. Later the periblast comes into intimate relations with the growing embryo; it lies directly against it, and ap- pears to receive cell increments from it at various regions; on the other hand, the nuclei of the periblast, from their intimate relations with the yolk, are supposed to subserve some func- tion in its assimilation. Aside from the question of periblast, the growth of the blastoderm appears not unlike that of the sturgeon. From the blastula stage to that of the early gastrula, the changes have been but slight; the blastoderm has greatly flattened out as its margins grow downward, leaving the segmentation cavity apparent. The rim of the blastoderm has become thickened as the ‘germ-ring’; and immediately in front of the dorsal lip of the blastopore its thickening marks the appearance of the embryo. The germ-ring continues to grow downward, and shows more prominently the outline of the embryo; this now terminates at the head region; while on either side of this point spreads out tailward on either side the indefinite layer of out- “LET aSug—Cappamas “ay yy Aq ‘ayy Wor) cazis peanqeu ‘snpors sajsrido1quay ‘sseq-eaQ—'S6 ‘OL] 138 Embryology and Growth of Fishes growing mesoderm. In the next stage the closure of the blas- topore is rapidly becoming completed; in front of it stretches the widened and elongated form of the embryo. The yolk-plug is next replaced by periblast, the dorsal lip by the tail-mass, or more accurately the dorsal section of the germ-rim; the ccelen- teron under the dorsal lip has here disappeared, on account of the close approximation of the embryo to the periblast; its last remnant, the Kupffer’s vesicle, is shortly to disappear. The germ-layers become confluent, but, unlike the sturgeon, the flattening of the dorsal germ-ring does not permit the forma- tion of a neurenteric canal. The process of the development of the germ-layers in Teleosts appears as an abbreviated one, although in many of its details 1t is but imperfectly known. In the development of the medullary groove, as an example, the following peculiarities exist: the medullary region is but an insunken mass of cells without a trace of the groove-like surface indentation. It is only later, when becoming separate from the ectoderm, that it ac- quires its rounded character; its cellular elements then group themselves symmetrically with reference to a sagittal plane, where later, by their dissociation, the canal of the spinal cord is formed. The growth of the entoderm is another instance of specialized development. In an early stage the entoderm exists in the axial region, its thickness tapering away abruptly on either side; its lower surface is closely apposed to the periblast; its dorsal thickening will shortly become separate as the noto- chord. In a following stage of development the entoderm is scen to arch upward in the median line as a preliminary stage in the formation of the cavity of the gut. Later, by the approxi- mation of the entoderm-cells in the median ventral line, the condition is reached where the completed gut-cavity exists. The formation of the mesoderm in Teleosts is not definitely understood. It is usually said to arise as a process of ‘ de- lamination,’ 1.e., detaching itself in a mass from the entoderm. Its origin is, however, looked upon generally as of a specialized and secondary character. The mode of formation of the gill-slit of the Teleost does not differ from that in other groups; an evagination of the entoderm coming in contact with an invaginated tract of Embryology and Growth of Fishes 139 ectoderm fuses, and at this point an opening is later estab- lished. The late embryo of the Teleost, though of rounded form, is the more deeply implanted in the yolk-sac than that of the sturgeon; it is transparent, allowing notochord, primitive seg- ments, heart, and sense-organs to be readily distinguished; at about this stage both anus and mouth are making their appear- ance.”’ The Larval Development of Fishes.*—‘‘When the young fish has freed itself from its egg-membranes it gives but little Renny. wt Fig. 99.—Young Sword-fish, Xiphias gladius (Linneus). (After Litken.) suggestion of its adult form. It enters upon a larval ex- istence, which continues until maturity. The period of change of form varies widely in the different groups of fishes, from a few weeks’ to longer than a year’s duration; and the extent Fic. 100.—Sword-fish, Xiphias gladius (Linneus). (After Day.) of the changes that the larva undergoes are often surprisingly broad, investing every organ and tissue of the body, the imma- ture fish passing through a series of form stages which differ one from the other in a way strongly contrasting with the mode of growth of amniotes; since the chick, reptile, or mammal emerges from its embryonic membranes in nearly its adult form. The fish may, in general, be said to begin its existence as * This paragraph is condensed from Dean's “Fishes Living and Fossil.” 140 Embryology and Growth of Fishes a larva as soon as it emerges from its egg-membranes. In some instances, however, it is difficult to decide at what point the larval stage is actually initiated: thus in sharks the excessive amount of yolk material which has been provided for the growth lic. 101.—Larva of the Sail-fish, [stiophorus, very young. (After Litken.) of the larva renders unnecessary the emerging from the egg at an early stage; and the larval period is accordingly to be traced back to stages that are still enclosed in the egg-mem- branes. In all cases the larval life may be said to begin when Dep ae y wy SBT PEEP ie a { vit f t { ee HTML WAS Fic. 102.—Larva of Brook Lamprey, Lampetra wilderi, before transformation, being as large as the adult, toothless, and more distinctly segmented. the following conditions have been fulfilled: the outward form of the larva must be well defined, separating it from the mass of yolk, its motions must be active, it must possess a continuous vertical fin-fold passing dorsally from the head region to the Vic. 103.—Common Eel, Anguilla chrisypa Rafinesque. Family Anguillide. body terminal, and thence ventrally as far as the yolk region; and the following structures, characteristic in outward appear- Embryology and Growth of Fishes 141 ance, must also be established: the sense-organs—eye, ear, and nose—mouth and anus, and one or more gill-clefts. Among the different groups of fishes the larval changes are brought about in widely different ways. These larval pecu- Fig. 104.—Larva of Common Eel, Anguilla chrisypa (Rafinesque), called Lepto cephalus grassii. (After Eigenmann.) liarities appear at first of far-reaching significance, but may ultimately be attributed, the writer believes, to changed environ- mental conditions, wherein one process may be lengthened, another shortened. So, too, the changes from one stage to another may occur with surprising abruptness. As a rule, it may be said the larval stage is of longest duration in the Cyclo- stomes, and thence diminished in length in sharks, lung-fishes, Ganoids, and Teleosts; in the last-named group a very much curtailed (i.e., precocious) larval life may often occur. The metamorphoses of the newly hatched .Teleost must finally be reviewed; they are certainly the most varied and striking of all larval fishes, and, singularly enough, appear to be crowded into the briefest space of time; the young fish, hatched often as early as on the fourth day, is then of the Fic. 105.—Larva of Sturgeon, Acipenser sturio (Linnzus). (After Kupffer, per Dean.) most immature character; it is transparent, delicate, easily injured, inactive; within a month, however, it may have assumed almost every detail of its mature form. A form hatching three millimeters in length may acquire the adult form before it be- comes much longer than a centimeter.”’ Peculiar Larval Forms.—The young fish usually differs from the adult mainly in size and proportions. The head is larger 142 Embryology and Growth of Fishes in the young, the fins are lower, the appendages less developed, and the body more slender in the young than in the adult. But to most of these distinctions there are numerous exceptions, and in some fish there is a change so marked as to be fairly called a metamorphosis. In such cases the young fish in its first conditien is properly called a larva. The larva of the lamprey (Petromyzon) is nearly blind and toothless, with Fig. 106.—Larva (called Tholichthys) slender head, and was long sup- of Chatodon sedentarius (Poey). posed to belong to a different Cuba. (After Lutken,) genus (Ammnocates) from the adult. The larva of sharks and rays, and also of Dipnoans and Crossopterygians, are provided with bushy external gills, Ricasanaanas y Weer oath VUINSNeSeassiaes Da Ne Nios aS My i} ), aN NY lic. 107.—Butterfly-fish, Chetodon capistratus Linneus. Jamaica. which disappear in the process of development. In most soft-rayed fishes the embryonic fringe which precedes the Embryology and Growth of Fishes T43 development of the vertical fins persists for a considerable time. In many young fishes, especially the Chetodontide and their allies (butterfly-fishes), the young fish has the head armed with broad plates formed by the backward extension of certain membrane-bones. In other forms the bones of the head are in the young provided with long spines or with serrations, which vanish totally with age. Such a change is noticeable in the swordfish. In this species the production of the bones of the snout and upper jaw into a long bony sword, or weapon of offense, takes place only with age. The young fish have jaws more normally formed, and armed with ordinary teeth. In the head- fish (Zola mola) large changes take place in the course of growth, and the young have been taken for a different type of fishes. Among certain soft- rayed fishes and eels the young is often developed in a pecu- liar way, being very soft, translucent, or band-like, and formed of large or loosely aggregated cells. These pecu- liar organisms, long known as Fic. 108.—Mola mola (Linneus). Very leptocephali, ied hea chown early larval stage of the Heeo tan called Centaurus boéps. (After Richardson.) to be the normal young of fishes when mature very different. In the ladyfish (Albula) Dr. Gilbert has shown, by a full series of specimens, that in their further growth these pellucid fishes shrink in size, acquiring greater compactness of body, until finally reaching about half their maximum length as larve. After this, acquiring essentially the form of the adult fish, they begin a process of regular growth. This leptocephalous condition is thought by Gunther to be due to arrest of growth in abnormal individuals, but this is not the case in Albula, and it is probably fully normal in the conger and other eels. In the surf-fishes the larvee have their vertical fins greatly elevated, much higher than in the adult, while the body is much more closely compressed. In the deal-fish (Trachypterus) the form of the body and fins changes greatly with age, the body becoming more elongate and the fins lower. The differences be- tween different stages of the same fish seem greater than the ‘ 144 Embryology and Growth of Fishes Fig 109.—Mola mola (Linneus). Early larval stage, called Molacanthus num- mularis. (After Ryder.) Fria. 110.—Mola mola (Linnzeus). Advanced larval stage. (After Ryder.) Embryology and Growth of Fishes 145 differences between distinct species. In fact with this and with other forms which change with age, almost the only test of species is found in the count of the fin-rays. So far as known the numbers of these structures do not change. In the moon- fishes (Carangide) the changes with age are often very con- siderable. We copy Lutken’s figure of the changes in the genus Selene (fig. 113). Similar changes take place in Alectis, Vomer, and other genera. The Development of Flounders.—In the great group of flounders and soles (Heterosomata) the body is greatly com- pressed and the species swim on one side or lhe flat on the bot- tom, with one side uppermost. This upper side is colored like the bottom, sand-color, gray, or brown, while the lower side is mostly white. Both eyes are brought around to the upper side by a twisting of the cranium and a modification or division of the frontal bones. When the young flounder is hatched it is translucent and symmetrical, swimming vertically in the water, with one eye on either side of the head. After a little the young fish rests the ventral edge on the bottom. It then leans to one side, and as its position gradually becomes horizontal the eye on the lower side moves across with its frontal and other bones to the other side. In most species it passes directly under the first interneurals of the dorsal fin. These changes are best observed in the genus Platophrys. . Hybridism.—Hybridism is very rare among fishes in a state of nature. Two or three peculiar forms among the snappers (Lutianus) in Cuba seem fairly attributable to hybridism, the single specimen of each showing a remarkable mixture of char- acters belonging to two other common species. Hybrids may be readily made in-artificial impregnation among those fishes with which this process is practicable. Hybrids of the different salmon or trout usually share nearly equally the traits of the parent species. The Age of Fishes.—The age of fishes is seldom measured by a definite period of years. Most of them grow as long as they live, and apparently live until they fall victims to some stronger species. It is reputed that carp and pike have lived for a century, but the evidence needs verification. Some fishes, as the salmon of the Pacific (Oncorhynchus), have a definite period 146 Embryology and Growth of Fishes of growth (usually four years) before spawning. After this act all the individuals die so far as known. In Japan and China Fic. 111.—Headfish (adult), Mola mola (Linneus). Virginia. the Ice-fish (Salanx), a very long, slender, transparent fish allied to the trout, may possibly be annual in habit, all the indi- viduals perhaps dying in the fall to be reproduced from eggs in the spring. But this alleged habit needs verification. Tenacity of Life.—Fishes differ greatly in tenacity of life. In general, fishes of the deep seas die at once if brought near the surface. This is due to the reduction of external pressure. The internal pressure forces the stomach out through the mouth and may burst the air-bladder and the large blood-vessels. Marine fishes usually die very soon after being drawn out from the sea. Embryology and Growth of Fishes 147 Some fresh-water fishes are very fragile, dying soon in the air, often with injured air-bladder or blood-vessels. They will die we Fig. 112.—Albula vulpes (Linneus). Transformation of the Ladyfish, from the translucent, loosely compacted larva to the smaller, firm-bodied young. Gulf of California. (After Gilbert.) even sooner in foul water. Other fishes are extremely tena- cious of life. The mud-minnow (Umbra) is sometimes ploughed up in the half-dried mud of Wisconsin prairies. The related Alas- Family Carangide. (After Liitken.) Fra. 113.—Development of the Horsehead-fish, Selene vomer (Linneeus). 148 Embryology and Growth of Fishes 149 kan blackfish (Dallia) has been fed frozen to dogs, escaping alive from their stomachs after being thawed out. Many of the cat- fishes (Stlurtde) will live after lying half-dried in the dust for hours. The Dipnoan, Lepzdosiren, lives in a ball of half-dried Fic. 114.—Ice-fish, Salanz hyalocranius Abbott. Family Salangide. Tient- sin, China. mud during the arid season, and certain fishes, mostly Asiatic, belonging to the group Labyrinthici, with accessory breathing organ can long maintain themselves out of water. Among these is the China-fish (Ophiocephalus), often kept alive in the Chinese settlements in California and Hawaii. Some fishes can readily ‘ eas Fig. 115.—Alaska Blackfish, Dallia pectoralis (Bean). “St. Michaels, Alaska. endure prolonged hunger, while others succumb as readily as a bird or a mammal. The Effects of Temperature on Fish.—The limits of distribu- tion of many fishes are marked by changes in temperature. Few marine fishes can endure any sudden or great change in this regard, although fresh-water fishes adapt themselves to the seasons. I have seen the cutlass-fish (Trichiurus) benumbed with cold off the coast of Florida while the temperature was still above the frost-line. Those fishes which are tenacious of life and little sensitive to changes in climate and food are most successfully acclimatized or domesticated. The Chinese carp 150 Embryology and Growth of Fishes (Cyprinus carpio) and the Japanese goldfish (Carassius auratus) have been naturalized in almost all temperate and tropical river basins. Within the limits of clear, cold waters most of the salmon and trout are readily transplanted. But some similar Fig. 116.—Snake-headed China-fish, Ophiocephalus barca. India. (After Day.) fishes (as the grayling) are very sensitive to the least change in conditions. Most of the catfish (S7/ur7d@) will thrive in almost any fresh waters except those which are very cold. Transportation of Fishes.—The eggs of species of salmon, placed in ice to retard their development, have been successfully trans- planted to great distances. The quinnat-salmon has been thus transferred from California to Australia. It has been found possible to stock rivers and lakes with desirable species, or to restock those in which the fish-supply has been partly destroyed, through the means of artificially impregnated eggs. The method still followed 1s said to be the discovery of J. L. Jacobi of Westphalia (about 1760). This process permits the saving of nearly all the eggs produced by the individuals taken. In a condition of nature very many of these eggs would be left unfertilized, or be destroyed by other animals. Fishes are readily kept in captivity in properly constructed aquaria. Un- less injured in capture or transportation, there are few species outside the deep seas: which cannot adapt themselves to life in a well-constructed aquarium. Reproduction of Lost Parts.—Fishes have little power to re- - produce lost parts. Only the tips of fleshy structures are, thus restored after injury. Sometimes a fish in which the tail has been bitten off will survive the injury. The wound will heal, leaving the animal with a truncate body, fin-rays some- times arising from the scars. Embryology and Growth of [ishes 151 Monstrosities among Fishes.—Monstrosities are rare among fishes in a state of nature. Two-headed young are frequently seen at salmon-hatcheries, and other abnormally divided or united young are not infrequent. Among domesticated species monstrosities are not infrequent, and sometimes, as in the gold- al Vet gee, 4 Fic. 117.—Monstrous Goldfish (bred in Japan), Carassius auratus (Linnzeus). (After Gunther.) fish, these have been perpetuated to become distinct breeds or races. Goldfishes with telescopic eyes and fantastic fins, and with the green coloration changed to orange, are reared in Japan, and are often seen in other countries. The carp has also been largely modified, the changes taking place chiefly in the scales. Some are naked (leather-carp), others (mirror-carp) have a few large scales arranged in series. CHAPTER xl INSTINCTS, HABITS, AND ADAPTATIONS HE Habits of Fishes.—The habits of fishes can hardly be summarized in any simple mode of classification. EL) In the usual course of fish-life the egg is laid in the early spring, in water shallower than that in which the parents spend their lives. In most cases it is hatched as the water grows warmer. The eggs of the members of the salmon and cod families are, however, mostly hatched in cooling waters. The young fish gathers with others of its species in little schools, feeds on smaller fishes of other species or of its own, grows and changes until maturity, deposits its eggs, and the cycle of life begins again, while the old fish ultimately dies or is devoured. Irritability of Animals.—All animals, of whatever degree of organization, show in life the quality of irritability or response to external stimulus. Contact with external things produces some effect on each of them, and this effect 1s something more than the mere mechanical effect on the matter of which the animal is composed. In the one-celled animals the functions of response to external stimulus are not localized. They are the property of any part of the protoplasm of the body. In the higher or many-celled animals each of these functions is spe- cialized and localized.